You're replacing a 2-man but mention only 1-man options; is that what you intend?
There is quite some difference between the two tents you mention--Seedhouse is double-wall, Contrail is single, for example. It's hard to compare things without knowing a little more about what you need and want, how tall are you, etc.
I've got the BA Seedhouse 1 (same dimensions as SL1 but heavier) and I don't like it much. For one thing, it's too short for me, but I'm pretty tall. It takes too many stakes to set up (11). The door is very hard for me to get in and out of (I much prefer side-entry tents). The vestibule doesn't hold much, especially for how much ground it covers, and anything it does hold is in your way for crawling in and out of the tent.
I also have a Tarptent Rainbow and love it. For me the benefit of single-wall construction (mainly, it's lighter) outweighs the detriment (needing more care to keep off wet walls). A lot of it is personal preference.