Another bout that is hard to choose one over the other. Both offer the same basic features of uploading files, syncing and sharing. I use both, but seem to use Dropbox more. Drive is great for collaboration with work and editing docs directly.
Dropbox doesn't offer editing of docs - you need to download, edit the upload again. You get 2GB of storage with Dropbox (which is less than Drive), but they have a pretty good referral system, so if you tell a few friends about it you can add to your storage limit easily.
I've used Dropbox Free for years and over that time slowly got it up to around 8gigs of free space from referrals. Then after getting my S3 and putting a ROM on it, I got the 50 gigs free for 2 years. That is crazy good for me. I do pay for 20 (or is it 25) gigs of space from Google Drive. My primary one is Dropbox though.
It fits well for me because I already use all of Google's other services. If I had to be specific about Drive vs others I would point to the fact that I can create things in Drive (Docs, Spreadsheets, Presentations, etc.) where you cannot do that with Dropbox for example.