If you're going to be storing lots of big files on your drive (such as full-length feature film videos), go for the bigger drive. If you'll only ever be storing things like Word documents, Excel spreadsheets, etc, go for the smaller one. (If you're a very keen photographer, taking many thousands of photos, and determined to use only your SSD for storage, you could also consider the larger drive but it would be just as easy to use memory sticks to store your photos on. That would enable you to opt for the smaller one. As an alternative to memory sticks, you could use cloud storage).
I'm typing this on a laptop with a 128 GB SSD and, for what I use it for, that's more than big enough for my needs. I'm also currently sourcing a new desktop computer for a friend, who'll only use it for word-processing, etc. So I'm not looking for anything more than a 128 GB SSD for him either.