That's not how it works. Mistings who get shot by aluminum bullets don't get their power drained from them, so why are you assuming that the Radiant's stormlight would be drained?
Alright, here's my current understanding:
Winner is based on four things: Equipment, skill, oath level, and situation. If a Mistborn is given unlimited Atium, a hemalurgic minigun loaded with aluminum spikes, Nightblood, and the bands of mourning, they are gonna win. If we match Vin up against The Lopen, Vin's gonna win. (On the other hand, if we match Taln up against Vin, he could probably kill her with a toothpick). Fourth oath is a huge step up from third oath, which is in turn a huge step up from second oath. Situation is, as has been brought up, of utmost importance.
So, all other things being equal, Mistborn with Atium has a huge advantadge over most orders of Radiant of the third ideal, an advantage over fourth ideal, and is unknown versus Radiants of the fifth ideal. Without Atium Mistborn are evenly matched with most third ideal Radiants, winner generally depends on the Knight Radiant's order. Fourth ideal, advantage goes to the Radiants, but the result still depends on the order. Fifth ideal, we have no idea.