With a good install, Sash packs should be comparable to their insert or full frame counterparts from a leakage and thermal efficiency. This is only as it applies to the operable section of the windows and glass.

A full frame window will allow the installer to address the lack of insulation in the weight pockets more easily as well as any air leakage between the window and the rough opening framing of the home. A sash pack will not. I might be mistaken, but I am not sure that the Sash packs qualify for the Energy Star tax credit.

On the glass loss issue, you should not loose much glass, if any, on a full frame replacement. You may get some glass back for that matter. The issue of glass loss is more commonly experienced on insert installations.

Uh-oh, looks like I'm mixing up my terminology. By full frame replacement, I meant use of a retrofit/insert window. Do the sash replacement kits compare well with those, efficiency-wise?

An insert window removes the sashes and puts a window that has a supporting frame and sashes in its place.

The biggest issue with sash packs is getting a qualified installer to put them in properly.

If the glass in the two windows is comparable, the efficiency of the sash pack (if installed correctly) should be relatively the same.

The issue with sash packs is that they are subject to air infiltration if not measured and installed properly.

An insert window can be sealed pretty easily so most mildly qualified installers can put the window in without any air leakage issues.

I am a fan of sash replacement packs, but on a 100 year old house, you and the installer need to evaluate the condition of your frames. Out-of-squareness due to settling can be brutal, it doesn't take a lot of that to make you regret using the sash packs. Also, variations in sash opening width between units that seem to be the same will kill you with "too tight" or "too loose" operation.
You can shim the liners if you have to but I would not plan to do that on more than one or two. I had a job go terribly bad because somebody insisted on the sash packs (no loss of glass) for historic preservation of frames and int/ext trim. Something to keep in mind.

I know of one company that has an "insert" wooden window which will forgive the slight out of squareness of an old window. It is Semco which is a old line window company out of Wisconsin. You will pay as much for this "replacement" as you would a new window but it is far better than any typical "replacement window" Since it also comes with it's own frame, you do not have to mess with sizing screens to the outside frame. Normally with a sash pack (Semco has them too) you cover the outside wood with aluminum coil and measuring for screens is a pain.

If the frames are good, not rotten, square and plumb, go for the sash pack. It is also possible to get sash packs that do not reduce the existing clear opening which could effect egress requirements. Even if your existing windows do not meet today's egress, you can replace them with like size but you cannot reduce from the current clear opening.