Definitely the future. Two reasons:

1) It's completely unknown, which would be extremely exciting. And with a little research, you could set yourself up to be very wealthy when you get back.

2) Going into the past is likely to be suicidal (Butterfly effect)

going back, I've heard how powerful my great grandpa was, i want to know about him and many more

Back in time.  There is a good chance the future will be dissapointing. 

I would go to the future to meet my grandchildren to see if I'd actually have kids later in my life and I've always been curious of the future and what the technology is like and stuff like that. There's also a chance that I would probably somehow mess something up in the past and I wouldn't want to risk that.

I choose both. In the next life one will be able to do both : )