Protocols: Both clients support MCCP, but only MUSHclient supports MXP. MUSHclient also allows you to add your own out-of-band protocols, while Mudlet only supports 102, ATCP and GMCP. As a server developer you can work around this to some degree - for example I send MSDP and MSP data to Mudlet using a custom ATCP package - but it's not ideal.
While it's true that Mudlet doesn't support MXP, it does allow you to turn text into clickable links. With a lot from help from Vadi, I was able to set up some triggers to catch MXP sequences and turn them into MXP-style links:
It did get pretty messy though, with some wierd display issues. In the end I found it easier to create a special "fake MXP" format specifically for Mudlet, and even then there are a few outstanding issues. For example I couldn't seem to stop it from stripping the colour, and in the end I had to send the RGB values with the tags.
Even now it causes some strange display problems when there's more than one link displayed on the same line, and I'm going to have to do some more tweaking before I release the script, most likely removing some of the links.
It's a bit of a hack job, and it's debatable whether it's worth adding such a special case to the mud for a single client. But as proof-of-concept, it's still interesting to note that it's possible. And from a playability perspective, it's kind of cute.