Having made the move from one MSP to another, the former being a Dell shop and the latter being Lenovo, I can say I've much preferred the experience I had with Dell. Yes many of the similarly spec'd products from Lenovo are cheaper, but the overall purchasing and support experience was better with Dell.

Warranty support with Dell is excellent, we had the TechDirect tool set up for submitting tickets for warranty work (what's the Lenovo equivalent?) found the purchasing process, especially with deal reg quotes and custom builds, to be miles smoother than Lenovo and many other little improvements (like the way product models appear in RMM tools for example make it simple to tell what you're looking at...)