900 Monster

I have the 600 and a mate has the 900. Love mine too much to sell but swapping bikes and riding his, the extra power is the difference to(@ approx 50mph) winding ours open, doing a quick look over shoulder to check its still ok to overtake and then go for it as the bike has picked up momentum, the 900, wind it open, quick look behind and your already starting to overtake lol. Not huge difference but in that instance very noticeable! Also got 6 gears as opposed to the 600's 5 gears plus an oil cooler and his(900) engine felt smoother running and better/smoother pickup. But with more power comes more speed and more temptation to use it hence why i prefer to have the 600 as i can use all its power and enjoy it without feeling like i need to go faster to get the most out the engine.
I'd say stick to the 600, the 900 does pull away from the 600 under full acceleration but it's not far behind....

 

Then, maybe your mate's 900 needs looking at because there's a world of difference between the two, especially at lower RPM.

My M900 is, admittedly, a bit 'tweaked' but it out grunts my 916 up to 6,000 rpm and is as quick up to 70-80mph at which point the, 30hp (but only 2lb/ft) more 916 will leave it standing.

 

As you say, yours is tweaked not standard. Nothing wrong with my mates M900, yes it is faster, it's gonna be, but that example was at 50mph, both bikes already at a cruising speed in their upper gears, it's the roll-on acceleration to overtake I was pointing out.
From a standstill his will takeoff and leave my 600 behind for dust. It has more grunt and isn't weasy when revving past 6k, but I ride my 600 Monster mostly 2k-5k revs and it's quick enough to leave most cars behind without pushing the engine.

 

I have a 900ss (same engine as the M900) and a Monster S4 (same engine as the 916) and I agree completely with what you wrote. The 900 doesn't have a lot of HP and it gets a bit breathless higher up the revs, but it has plenty of bottom end punch.