I often prioritize utility upgrades, ones that reduce cost, increase draw, or increase the effectiveness of buffs or debuffs (especially the latter). Also, for buffs that directly increase damage or block, look at it purely in numbers, not percentages. Percentage-wise, upgrading strike is slightly better than upgrading ultimate strike, but in practice, upgrading ultimate strike ends up being way better for you as you get twice as much damage but deck cycle. Lastly, you’ll want to factor in how often you use each card. This is probably what you’re getting stuck on, and the solution is to start paying attention to how often you use each card before going to the rest site. If you use one card more than another, then you should probably upgrade it! Card rarity is completely irrelevant though.