Hi Jon! Welcome on Unilang!
I learned Ukrainian first, and only a few years later, when I already had a solid basis, I picked up Russian. Of course it sometimes happened that I mixed up things, and sometimes I probably also said something the Ukrainian simply because I hadn't learned yet that it was said differently in Russian, but in general I would say mixing things up was not a big problem. On the other hand, they are similar enough that knowing one surely helps learning the other. Of course, it could be different for you, since you're probably less advanced in Russian than I was in Ukrainian when I started Russian. You can just try it. If you find it too confusing, you can always quit Ukrainian and try again in a year or two years' time. In any case, them most important thing, in my opinion, is that if you learn several (similar) languages, is that you keep all of them active. I didn't do that in the last two years, when I've been speaking Polish all the time and hardly any Russian and Ukrainian, and as a result I now speak Russian/Ukrainian full of Polonisms.
About which one is easier: I don't think the difference is very big, but if anything, I would say Ukrainian is easier. Especially the pronunciation, but that might be very different for an English speaker, of course. Back when I started learning, it was rather difficult to find materials for learning Ukrainian, so in that respect learning Russian was easier, but nowadays there is much more and it's also possible to find Ukrainians on the internet in case you need help/practice, so that isn't a problem anymore.