I personally would go with a 256GB, you get twice the space and the price difference right now between the two is close enough that it's a good buy. And no an SSD's performance does not drop if it is half full, I think you want to have somewhere between 7% and 14% free space on an SSD to ensure performance. Even at completely full though I would suspect the performance is much better than a typical HDD.
On top of that if you get a drive that is 120GB, 240GB, 480GB...these already have space partitioned off so that you do not need to worry about losing performance when filling up the drive.