The R21 Ultra is the newer base and the cheaper one, which almost never happens. Both put out the same 21 Nm, and on the rubric they are near-identical twins - the only axis that separates them is value, and that is because the Ultra undercuts the original R21 while bringing a newer motor architecture to the party. Choosing between them today, the straight answer is that the Ultra is the one I would buy, and it is not especially close.
The newer motor is the reason to care beyond the price. Twenty-one Newton-metres is 21 Nm either way, but the Ultra delivers it a touch more smoothly, with the fine detail sitting cleaner on top of the big forces. It is not a night-and-day gap - the original R21 is still a properly capable high-torque base and I have no complaints about how it drives - but back to back the Ultra is the more refined of the two, and it does it in a more compact package that is easier to live with on a crowded rig.
So when would you take the original R21? Really only if you find one cheaper on the used market, or you already own one and you are weighing an upgrade - and if that is you, I would not rush, because the Ultra gains are refinements rather than a reinvention. For anyone buying fresh at 21 Nm, though, the Ultra gives you the newer motor, the smaller footprint and the lower price all at the same time. That is the rare upgrade that costs you nothing to take.