Either will be absolutely fine. In general terms a 'mark II' model only has incremental upgrades (excluding the Selmer SA80 I and II) - and it's often the case than many players consider them retrogrades (see Yamaha YAS62).
If you can play both horns side-by-side and detect a preference, go with what you feel.
If you're buying on spec, get the horn that represents the best deal (price/condition).
If the 900 models are particularly cheap, they'd be worth having.
In short, anything by Yanagisawa is a good bet.
And if you can run to it, the 991 is worth the extra. It's very similar to the 901, but just seems to have a little bit more of everything.
I'd steer clear of the 'U' model though. Unlacquered horns can be difficult to look after if you're a wet player.