I was just playing a few games and going into some Queens Gambit declined lines when I started wondering if there would be some sort of theoretical difference on which piece I opened up with first, even though I would still be achieving the same position. Obviously the most common first move is d4 but if I were to play Nf3 what would the differences be?
1st move [pawn vs knight theory]
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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).