Thanks for the reply alexb. I haven't shut the door on the 128i yet, and while I think the sports package would probably remedy my issues with the seats (the base seats are just... not very good... the seats in my GTI are 10 times better), the steering was a big surprise. I love the weight, and I love the balance of the chassis, but it just doesn't feel as "tight" as the GTI. That's a big deal to me, since I basically want something sportier/tighter than the GTI. I just really need to get seat time in a 128i or 135i with the sports package, really.
The BRZ is definitely a step down inside, and in power. But it feels like a proper sports car. I felt the seating position was better than in the BMW, and in handling, there really wasn't any comparison. I wish I could mix the two cars together to make a bastard of a vehicle, but that's not an option, apparently. :-)
And yeah, the GTI has held its value well. Probably better than the residual on my lease, which opens up some doors, but I'll probably be lazy and just turn it in, ha.