Assuming it's the same model, moving from 650 to 900 or even higher would gain you
vaping time before a recharge is required. I tend to use a 900 or 1,300 model depending on my mood, which in my case gives me up to about 12 hours of vape time (I'm a heavy vaper).
Additionally, the larger models, needing recharging less often, will tend to last longer. Lithium
batteries only have so many recharges during their lifespan, so if you fully drain your 650 it'll happen faster than a fully drained 900. That extends their life.
Plus if you get your 900 back on the charger before it fully discharges, it extends the lifespan even further. Partial charges on lithium batteries don't subtract even the percentage of full charge you gave them. I remember reading that if you discharge a lithium battery halfway you'll get MORE than three times as many charges as if you used it fully. That's more than half again the lifespan and power.
However, if you're happy with your 650 the only thing I'd suggest is having a backup just in case it dies...