<p>In the lab I'm working in, I don't really know the PI, but I work a lot with one of the investigators (a fellow). I was just wondering how a LOR from the fellow would look, seeing as how it would be much better and more personal than a letter from the PI.</p>
<p>Hm. One thought: have them cosign it?</p>
<p>I had both my PI and the postdoc I work with write me letters, though both new me reasonably well; there’s no harm in having two letters from your research experience. If for some reason you must only choose one, I think the fellow would actually be the better choice if it gets you a much more personal letter.</p>