Well if you play 1. Nf3 you give Black more options and won't necessarily get into a 1.d4 opening. Instead, it could become an English after 1.Nf3 c5 2.c4 or a Sicilian after 1.Nf3 c5 2. e4 (usually 1.Nf3 players go into the English though)

Also, playing 1.Nf3 means you cannot play  Nimzo or a 'good' QGD Exchange (with the Nf3, the plan with f3 and e4 can't happen). Theres also some extra Benoni lines Black might throw at you like 1.Nf3 Nf6 2. d4 c5 3.d5 b5!?. All in all, theres quite a few differences and you need to know more to start out with 1.Nf3. That being said, it is a very flexible move.

1.Nf3(the Reti) is a flexible move that is used pretty widely by GMs. However, Black has chances to challenge the centre. In internet, high-speed, super-tactical games, I would recommend 1.e4 or 1.d4. Personally I hate the Queen's Gambit Declined, mostly because I'm not familiar with the theory. I prefer QGA, or even better, one of the Indian defences(those are old hat for me). 

Yes, 1 Nf3 can very often transpose into QG positions, and in certain cases advantageously - although usually it is just a simple transpo.

It is more flexible than 1 d4, since you can still reach Reti, English, and other openings like KIA.  However, there is Yin and Yang in all things, and flexibility for White means less pressure on Black, so the net result is 1 Nf3 is less likely to result in White achieving a "normal" opening advantage than he might by the straightforward 1 d4.

What is far more important than the difference in the opening advantage which Carlsen or Kramnik might force is what you as a player feel more comfortable and confident playing.  You aren't those guys, but you won't be playing Aronian, Anand, or Ivanchuk, so your mileage will definitely vary.

Play what you feel suits you and your style best, and forget theory's current recommendations - in the long term, you will have better results and more fun by doing what you enjoy.