I used to always leave the fat cap on tri tips and figured it would help retain some moisture since almost all of the tts I was buying were select grade, and fairly lean. The last one I cooked was a fairly lean piece of meat as well, but I completely removed all the fat cap and the silverskin. I reverse seared it using a few pecan chunks and "ridge" charcoal briqs on my Weber OTS. Took it to 132 during the sear and cut it correctly. It was out of the park good with plenty of moisture and wonderful flavor.
Depending on how much fat was left on the untrimmed roasts, I'd make the decision on which one is the better deal. If the untrimmed ones looked better (more marbling, etc), I'd go that route and trim it myself. If the trimmed ones looks better, then I'd go that route. Sometimes a piece of meat just calls my name, and I'll reach for it. But I really liked the way the fully trimmed one turned out using reverse sear and keeping it med rare all the way through.
And cutting these things correctly makes a huge difference in tenderness. The youtube video showing how to cut the roast at the "seam" then cutting both pieces across the grain, and on the bias is the way to do it. You'll see many videos of people cutting tri tips in various ways, and I'm convinced the "seam" method is superior.
Modded Brinkmann Gourmet. Basic UDS. Rescued 22.5" OTS. SJS Mini-WSM. Stubbs. B&B Oak Lump.
Cannondale 2009 F8 (for burning off all the great BBQ)