I looked at a lot of trailers earlier this summer. Sorry to say though I was really looking at 2 and 3 place ones so I'm not up on overall length of the 4 place ones. As I'm sure you know, the inline will be longer than the side by side deckover. I think you need to ask yourself what style you need. The inline is easier to see around when towing and probably easier to tow, but it all depends on what you pull it with. From my experience the inline maybe more expensive if you are looking in the used market. Not sure about cost comparisions for new ones. If your limited on your overall length in your storage area, the deckover side by side will work better, but the opposite is true if you have width limitations. Also, the inline may be short enough to get through a standard garage door, but the deckover may be too tall.
I really agree with whitedust, don't plan on the 121 being your sled a few years from now. The reason I looked so much this year, is because I picked up a use 128 and it was too long for my old trailer. When I bought my old trailer (a 10' deckover enclosed) the guy told me he could have sold it 20 times before I bought it if it had been a 12 footer. I thought, no problem I'm sticking with these sleds, and if I don't, I'll just buy a sled that fits the trailer. That was only 4 years ago and I already had to change trailers. Find something that gives you a little room to grow and make it work in both the space you have available and your budget.