For an undergrad, which is more important? LoR or publication
In the US, good letters of recommendation are a must. Research experience, especially resulting in publication is definitely a plus, but since it is still a bit rare, you are probably already ahead of the curve there.
The &quot;problem&quot; with undergrad research is that, given the time (and maybe money) constraints it is hard to reach a conclusion on anything that is very significant. In research, you don't know the answers when you start and you are looking at the unknown. There is no schedule that you can apply to actually reach finality unless the problems are rather straightforward. The time between writing the paper and publishing it can also be long and indeterminate if the journal is reputable. Conference publication is a bit more time predictable, since the conference will be held on a certain date, but if you miss a deadline, the next opportunity might be a year away.
If you are doing something hard, then my recommendation would be to keep at it. People will, I think, reward you for that even if you don't reach a conclusion, provided that the advisor can praise your efforts and predict your future success.
I think you are already in a good position on this point, though doctoral admission is dependent on many things beyond the two you mention. But, in the US, letters are essential.