For graduate admissions, focus on bachelor thesis or other research publication
Assuming both are closely related to your subject for graduate studies, either could be more important.  Even if you have a published paper, graduate admissions committees are not likely to truly read it, though many people may take a quick look at it, to get some sense of what level it is at.  (Though if this is a joint paper, just from looking at the paper, a committee will not know how much is due to you.)

Here committees will form an opinion largely based on your letters of recommendation, so if you impress someone in the research lab or your thesis advisor, you should definitely ask for a letter of recommendation.  If your thesis advisor is well known, a strong letter from that person can make a bigger impact that one from your research lab.  On the other hand, if you don't do a good job on your thesis, you may want to think twice about asking that advisor for a letter.

So, ideally, you should try to do a good job on both, and get letters from both.  Presumably you have some tacit agreement with both the lab and your thesis advisor for how much work they expect from you, and you should try to meet both of those.  If you can't, this means you are overcommitted, and you need to have a conversation with one or both of these parties.  (The same goes if you are unclear on how much work is expected of you and you are having trouble balancing these projects.)