Where are you located? The clear air space will be more efficient but I think you need to look at both before you decide up or down. I agree with eastbay that based on the operator type, you are much less likely to see it.

Most Marvin retailers will have glass package samples or look for a Marvin Window and Door Showplace.

Yes and at the very least should have a sash both with and without the spacer bar. The showroom I had previously worked in had one upper sash and one lower sash with each type in it for just that purpose.

I'd recommend the spacer. If you are spending the $$$ on the SDL's in the first place, do it right. The drop in efficiency is not that much.

The purpose of the spacer is to give the affect of an actual "True or Authentic" divided lite and if it is carried off, it can trick even the best. As I said earlier, it is not needed on a Double Hung especially as you seemed concerned about the difference in efficiency. Here in California I advise my clients to almost always pay the minimal up charge for the spacer.

Here's a photo of a Marvin window with the spacer. Depending on your point of view, I think they would look very odd without a spacer.

I personally tend to agree with lkbum, without the spacer bar they just don;t look right. Am I right, am I wrong, don't know just my .02 cents.....

precisely as eastbay explained. It is for appearance, and I agree with the others that it just doesn't look right without it. You are paying top dollar for a beautiful window and SDL option, this is not the area to skimp IMO, or to trade off for efficiency if that is the main concern.

We used the same combination of windows in our house, it has worked out quite nicely. The windows look odd without the spacer bar, it is definitely a worthwhile addition and the efficiency cost is pretty minimal. We had no problem hitting better than stretch code energy requirements with these windows. The U factors for the Integrity windows seems to be the same with and without spacer.