As a nvidia miner (60x 1070), i can tell you they perform very well, but the problem is your brand of choice, i don't know about msi when it come to amd cards, but nvidia card branded MSI are total garbage, go with EVGA and you'll see it's much better
After i can understand not willing to change from AMD to nvidia, simply because you know the amd's so well, when you have such a big farm you have alot less time to look @ the new hardware and try again to find the best algos, best overclocking sweet spot, optimal rig building and such.
I encounter the same dilemma upgrading my current farm adding amd rigs (with the coming of vega), the nvidia cards are now with alot of new optimised miners coming, very strong on alot of alternative algos, they do very well also in zcash, pasc, and others, not too shabby in ETH & XMR but who care since it hasn't been profitable for weeks against other algos.
They also had for me that started mining not long ago, more "business security" if something went extremely wrong i know i could resell them very fast and at good price (added that i had a also a very nice buying price), because gamers look for nvidia card and not amd's nowadays.
As ROI go, i wouldn't probably have a real objective advice on it, because i had such a sweet deal on my gpu's that it wouldn't reflect the true market today (i basically had a 30% rebate counting all the offers i cumulated on them), also 4 years warranty. For the moment after 6 months of mining, im pretty happy and already @50% ROI and my electricity before last month was twice as expensive, i expect to finish ROI in 4 months with my current situation
Also i never use nicehash, you can get so much more money out of your rigs if you deal with the coins directly, trading (especially selling @ the right value/moment) even lightly give you more money each month (in my case 10-15% more easy)
To sum up, give it another go in the future if you got time, but please no msi's