Not only should you use both, you should use MCP for
way more than Sneak Toggle. The Morrowind Patch Project (MPP) addresses things like quest errors, and inconsistencies in the game's content by providing records that modify the Morrowind, Tribunal, and Bloodmoon .ESMs. The Morrowind Code Patch (MCP) addresses things like savegame corruption, crashes, and so forth (check the MCP thread for full details) by patching the Morrowind.exe file with the options that you choose.
It is good practice these days to be using both, but note with the Code Patch that all changes are optional. The bottom section in the patcher, the bugfixes, are extremely recommended. The upper sections address some gameplay issues, map size, and a number of other things. Again, see thread (or readme) for the importance of each.
Also, if you're going to be using a Sneak toggle, I'd recommend the Code Patch version over a dedicated mod... it just makes things simpler to have it built into the .exe, you can't break the script and it has no MWSE/MGE dependency.