4" cans are better. Just make sure what lamp you choose has the type of spread that your space needs.

I also like the 4" cans because with the larger ones, the ceilings tend to get a little busy... Especially when you put a lot of them. I think most people tend to put more recessed cans in than is really needed...

John, I agree that people tend to install way more cans than necessary...

In our case, I'm hoping that limiting the design to four cans, at opposite ends of our long entry will be just the right amount. The two green "+" signs (without squares) will be MR-16 accent lighting for art work and fireplace.
The black "+" is currently our only source of ambient lighting in the entire living room (not enough!). We also have two lamps which flank the sofa across from the TV. I attached a wide angle picture of the room for reference.

I think we all agree that 4" cans are more attractive, but for our lighting lay out (with just four LED cans being added), will the lower wattage of the 4" be enough to sufficiently light this whole 2nd half of the room?

Oh! and all the lighting in the room will be controlled on dimmers!

I'd put the 6's for coverage with only four cans. honestly, I don't think most people notice "style" in recessed lights, unless maybe they're eyeball finishes (and I wouldn't bother to switch out those at our house either, heh). Check whether the bulb wattages (or lumens) really differ in the store you use, though, as it might not matter. More important will be how deep in the can the lightbulb is -- those that are right at the surface (common with LED fixtures) disperse better than those with deep lights, which make more of a single downward beam.

I'll add another vote for 4".

Thanks to everyone weighing in!
@Archi/Build Group and @John Barton - are your recommendations for 4" cans in general, or specific to our install plan?

The shortest distance between the cans will be 3ft 10in; the longest distance between cans will be 11ft 4in... But! If you account for the MR-16s over the fireplace, (even though they are not providing the same *type* of lighting) we should still be getting 'OK' coverage between the LED cans, right?
I read a rule of thumb that says it's 4" cans for 4ft spacing, 6" cans for 6ft spacing (etc, etc) but I keep getting thrown off by the unlit section in the middle! And I don't want to add another can here since there's nothing worth highlighting in this part of the room...

In general, due to the technological advance of LEDs. I just employed 2" cans to great effect on a remodeling where just a few years ago 6" cans were required. As to your question of "ok" coverage, I would need to see a plan with furnishings and know your intent with relation to the recessed lighting to answer that. The metrics of the LED and the specified recessed fixture are also important.

It is cool how quickly LEDs, and lighting in general is progressing. Our intent with the recessed cans (other than bringing light into a section of the LR that an night receives virtually none) is to highlight the piano, and our lovely broyhill bar cart. There's not a whole lot else in this entry. The fireplace, of course will get the treatment of the MR-16s, so we're not too focused on using cans to accentuate this. Here's a link to the rest of the photos from our Houzz Project to help understand furniture layout, etc.
http://www.houzz.com/projects/567642/living-room-design
I think I'm leaning towards the 4" cans. I do like the more sleek look (although, I agree with acm, it's not a drastic difference).. But I still can't shake my concern that with dimmers, we shouldn't just go for the biggest wattage option.

To determine if your specified design is adequately lighting these two items, you will need to look at the metrics of the recessed cans and LEDs to determine if there projected spread, luminosity, and color will achieve the desired results.

Sounds like I need to make a call to my installer! He favors the 6" cans, but obviously hasn't quite convinced me on that choice. I'll get his specs for projected spread of each sized light, and make my final decision. Thank You!!