A post by Iocun on Achaean forums:

Mudlet:
– Free
– Very fast
– Multi-platform
– You can do pretty much anything with it
– Under constant development
– You can make it look really pretty
– Scripting is based on a non-proprietary language (Lua), meaning you can use the acquired knowledge for other things as well
– A helpful community, both on the Mudlet forums and in its Achaean clan
– Really not all that difficult to learn (but that’s hard to generalize. I can’t speak for everyone.)

CMud.
– Costs
– Much slower
– IMHO inherently less pretty
– Still rich in features though
– Is primarily intended to be used with its own scripting language, making the knowledge less applicable for other things, but offering the advantage of being easier to learn if you already knew zMud. I think you can use Lua in it as well though.
– Again: Easy to learn if you know zMud.
– Not all that difficult to learn initially either
– No idea about how helpful the in-game clan is, having never been in it, but it’s still a widely spread client for which you will be able to get plenty of help.
– Better in-client documentation than Mudlet.

Both are decent clients, but personally I don’t know why you’d want to pay for a client that does the same things slower and uglier than a free one. I have some rather complex and big scripts which I couldn’t even have dreamed of running in zMud. Cmud would have made it somewhat more feasible, but some of my scripts (mostly my own curing system and mapper) would still bog it down horribly.