Gustavo, 

The Select GoLo will have a slightly larger head, which helps to increase the MOI for that putter.  Both are great options, but keep in mind the head size when making your decision.

Cameron

1-888-TITLEIST  

I use the the GoLo S and love it.  Very smooth off the face and the larger head with the increased weight really gives me a lot of feel on fast pace greens. 

I have both and the GoLo's in the bag now. The Fastback had way too much going on for me (alignment/sight lines). At impact, there's a slight metal pinging sound and I didn't like that. With the GoLo, it sets up solid. Looks clean at address. Great feel/feedback. 

Everyone's different though. I know a lot of people who have the Fastback and absolutely love it. I also know people who don't really care for the GoLo. If it's possible, you should go to your closest authorized Titleist rep and try them out. It'd be even better to go through a fitting. The toe flow and offset will differ with each one. Of course, you want to make sure that you have the correct length, loft, lie, grip, etc..