I really don't know if the extra 1/4" would matter - and it might have just depended on who was measuring the seat, and where. It's probably at the widest point with the base on, but where that widest point is matters a lot depending on what seats are next to it. It also matters if your centre seat is raised up on a hump or not, and how scooped the outboard seats are. What I mean is the various elevation of the vehicle seats means a tight 3-across would work in one car with a particular set of seats but not in another if the widest spots don't puzzle in well with the seat next to it. If both work I too would get the Snugride 30 because it gives you more weight (the same height though), and is generally cheaper.
Unless someone has done one in your exact car, there's really no way to know for sure except to go and try it out. Are you near a Babies R Us? They let you try seats and would have both of those seats. Try different orders, as sometimes that matters greatly. You must install the centre seat with a seat belt, and doing the outboard seats with the seat belt too will likely give you more room again as you can shove the seat to the side before tightening. All three need to be independently tight and not relying on the one next to it for a tight installation, with less than 1" of movement at the belt path.
