I can't really compare because I haven't used a 128i with power seats but my convertible has the sports package and manual seats. They are, as you note, very adjustable. If you remember to pull the latch at the top of the seat and use it to slide the seat forward you may be able to do this with the one operation. It seems like we often end up using the lever for sliding the seat, however. But when you do it right, the seat goes back to the same position. It is not hard. But I do not use the rear seat much. It is not uncomfortable as long as the front seat passenger gives you some leg room. My 6'4" son sits in front of my 5'9" daughter sometimes. But he is further forward than he prefers. She complains about the wind if the top is down. It is a viable option but we often just take the SUV when there is more than 2 of us. For 2, the bimmer is definitely preferred.
I agree about the memory comment but I also am the only driver of my bimmer so it was not a factor. In 2009 when I bought mine I think the power seat offered a lumbar support but I haven't needed it. The sport seats are very comfortable. Even with the manual seats, the bolsters adjust electrically.