I don't think you can go wrong with either one. We have both. Both are very roomy. I can't believe how long the SL2 is and its a little more narrow so it fits in tighter spots. If you are tall, I would probably recommend the SL2. The volume inside the SL3 is amazing. We call it the circus tent. I would say it is more livable and a better choice if you are stuck inside from bad weather.
I like both better with just the bathtub floor instead of the 'nest'. The mesh in the nest eats up some of the interior volume and make them feel a little more cramped. If you need bug protection and plan to use the nest most of the time, then I would probably lean more towards the SL3. Our nest for the SL2 is much shorter than the fly (by about 1.5 ft). So, this negates the extra length and makes it trickier to get in and out. However if you are going to use it with just the bathtub floor, then I think I would lean more toward the SL2. Its a little easier to pitch and probably a little lighter. But like I said, you can't go wrong with either.