Jennifer,

There are many factors that I would consider in choosing varieties of pears or cherries. The rootstock as well as the scion variety should be considered. Factors like amount of pruning needed (and your pruning skill), disease resistance (and possibly pest resistance), planting location (dry or wet area, windy or protected area, etc), productivity (how much fruit you would like from each tree), usage (fresh eating, cooking, storage, etc), pollination requirements (generally you'll need at least two compatible varieties to pollinate each other), and taste are but a few of the factors that usually go into consideration when choosing a variety.

The reasons that so many varieties exist is that different choices are best for different situations and different preferences. There really are not perfect choices, and what's right for you, may not be what others have chosen. When I picked out my trees, I concentrated first on disease resistance. After choosing possibilities that would be more likely to avoid some of the most common disease and problems, I narrowed the choices down from there.

The primary disease associated with pears is fireblight. I would strongly urge you to ensure that the varieties you choose are resistant to that disease. Other common diseases/problems that affect pears include pear decline, leaf spot, cedar-apple rust (don't let the name fool you), scab, black rot, and bitter rot. For a more complete list, take a look at this list of pear diseases: http://en.wikipedia.org/wiki/List_of_pear_diseases

Prunus (the genus that contains cherries) is a fairly disease-prone genus. Sweet cherries are even more prone to problems than sour cherries are. Bacterial blight/canker and rain cracking are among the most common cherry issues.
For a more complete list, take a look at this list of cherry diseases: http://www.apsnet.org/online/common/comment/cherry.asp

I will try to write more later. This is just a fraction of the tip of the iceberg. But before I go to bed, I'll give you a few varieties (there are many many more) that you might consider based on disease resistance / common-problem avoidance:

(European) Pears:
Magness
Moonglow
Potomac
Seckel
Warren

Semi-dwarfing & Full-size Pear Rootstock (hopefully, more on this later):
OHF87
betulifolia
OHF333

Sweet Cherries:
Black Gold
Black York
Regina
Sam

Sour Cherries:
Montmorency
Northstar
Surefire

Semi-dwarfing & Full-size Cherry Rootstock (hopefully, more on this later):
Mahaleb
Mazzard

Selection of rootstock is very important. Rootstock determines or influences tree size and architecture, fruit size and quality, disease and pest resistance, drought resistance, soil adaptability, cold hardiness, proclivity to sucker, graft compatibility, amount of fruit produced (productivity), how young the tree will produce (precocity), and how long the tree will last and produce fruit (senescence). The selection of rootstock will affect things like how much pruning will be required, how you train the tree, and whether the tree will need support (many rootstocks will not reliably physically support the tree without external support).

When choosing a rootstock, size (amount of dwarfing) is usually the first characteristic considered. All rootstocks have some advantages and some disadvantages. There are no perfect rootstocks. Smaller (dwarf to semi-dwarf) rootstocks are the most popular choice for the home gardener and becoming increasingly popular among commercial growers. Smaller trees require less pruning, are easier to manage (harvest, prune, spray, etc), are generally more precocious, and fit into smaller yards. Larger rootstocks (larger semi-dwarfs to full size) are sometimes more disease resistant, frequently more productive per tree, generally more drought resistant, more likely to be self supporting, and usually produce a longer-lived tree. I usually choose larger semi-dwarfing rootstock because I have plenty of room and, for my particular application, consider senescence probably a little more important than many growers.

You may want to narrow down a list of possible suppliers for your trees and consider the rootstocks they offer. If you'd like, I'd be glad to help you review a particular rootstock once you've narrowed your selection down a little (at least size range).

When choosing a supplier, you'll probably want to consider product availability, reputation, and price among other things. You may be surprised how hard it is to find a good single source for everything you want. Often compromise or use of multiple suppliers is required.

When I consider suppliers, I compile a list of ones with good reputations and then narrow my list further based on availability. I always use Garden Watchdog as one source of review. If the suppler doesn't have a good Garden Watchdog rating, they can forget about getting my money. I also further review suppliers based on things like comments from other growers here on Gardenweb.

Some suppliers you might consider are:
Adams County Nursery (excellent GW rating)
Burnt Ridge Nursery (very good GW rating)
Cummins Nursery (excellent GW rating)(this has been my main source for pears and cherries)
Edible Landscaping (pretty good GW rating)
Raintree Nursery (pretty good GW rating)
St. Lawrence Nursery (very good GW rating)
Trees of Antiquity (excellent GW rating)

You can find other suppliers from searching the Garden Web Fruit and Orchards Forum and on the NAFEX list linked below. You may find that some companies listed by NAFEX are no longer in business or that some of the website links from the NAFEX page no longer work. I contacted NAFEX months (years?) ago and asked if they would like me to provide them with a completely updated list, but they never bothered to respond. The list can be somewhat useful with a little research.

Here is a link that might be useful: Somewhat Outdated NAFEX Fruit Tree Source List

Helpful Links (mostly about rootstocks):

Cummins Nursery Rootstock (pear rootstock info at bottom of page):
http://www.cumminsnursery.com/rootstocks.htm

Trees of Antiquity Rootstocks (some pear and some cherry rootstock info):
http://www.treesofantiquity.com/index.php?main_page=page_6

Dave Wilson Nursery Rootstocks (info on pear and cherry rootstocks):
http://www.davewilson.com/roots.html#mahaleb

Oregon State University Sweet Cherry Rootstocks and Varieties (various useful info):
http://www.utahhort.org/talks/2002/LynnLRV.pdf

New South Wales Pear Rootstock (some useful info):
http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0016/120229/pear-rootstocks.pdf

Brandon, thank you so much for all the info! I truly am a newbie...I had no idea there was so much to consider. This was really helpful. I'll check back as I narrow it down.
Jen