I use both and have nothing but positive things to say about both companies. I have needed help a few times at Namecheap and without exception found their assistance prompt and helpful. Namesilo I have used less, and have far fewer domains with them. The interfaces are far different, but I like both fine.
I think that the choice will largely depend on the extensions you hold. Namecheap have a much broader set of ngTLDs and generally better year one prices in new extensions (and renewal prices on some). Namesilo normally have better prices in the legacy extensions, and sometimes for renewals and transfers of other extensions.
A major advantage of Namesilo if you deal in com/net/org/info/pw/biz is that they are a fast transfer registrar to get Premium Network coverage at Afternic listings.
Namecheap have some great promotions at various points in the year such as their transfer day in the spring and their end of November annual deals. On the other hand, Namesilo pricing tends to be more consistently good.
They both have marketplaces for selling domain names. I have used Namecheap a lot, and plan to start using Namesilo. As others have pointed out the commission is a bit less on Namesilo and also payment options more flexible, although possibly Epik beat them both.
I tend to sell to end users that are essentially single person companies. Namecheap offer hosting services, so some end users prefer to buy at there and to be able to quickly and easily get hosting up and running (of course GoDaddy also have this advantage and are much better known).
If I handled only com/org/net/info I would probably have all of mine at Namesilo. If only ngTLD probably only deal with Namecheap. If mix of both, I think you should use both.
Bob