I guess it's like asking if ipod or blackberry or n900 or htc etc.
While there are vast differences in how it works under the hood, for the most part you can expect about the same functionality, and it comes down to the provider and a particular feature you prefer in one over the other. In this case instead of your carrier you need to worry about you host, whether they offer the php version required aand the database you need etc. But for most part, you should be fine.
Modx Evo requirements : http://modxcms.com/learn/general-requirements.html
Modx Revo requires: http://rtfm.modx.com/display/revolution20/Server+Requirements
TextPattern requirements: http://textpattern.com/about/119/system-requirements
I find this an important thing to start with, while my server can handle either, you never know what server your clients are running(there still are some old configurations out there)
I haven't used Revo much yet, but it looks like there are quite a few really nice enhancements. One of them is installing packages. Previously you have to create TVs snippets chunks, so installing a package wasn't always so straight forward.
Modx Evo however has the import html feature, where you can load put your html site in a folder set the tag that holds the content, and modx will automatically create and fill the resources(pages), pretty nifty, but I've only used it to see what it does :)
Modx lets you get away with not knowing any php, but as your requirements change, you might find yourself needing an extra feature of script that is easy to add. I've found that using modx for the past 2 years or so has vastly improved my php abilities because of these additional functions required.
While I haven't worked with textpattern much I do remember installing it and playing around, but it just wasn't love at first sight, like with modx.
I also think modx is pretty easy for clients to navigate with some minimal coaching, but that is more of a comparison with joomla and drupal etc.
Bottom line, I think few people have spent enough time with both to REALLY understand both in terms of differences/advantages and mostly what you'll get is "the one I use is better; evidence being me using it" (myself included) or worse yet "I see you're mentioning CMSes, here's my favorite one"
To wrap up, one more important aspect (and this one is really important if you don't know your way around backends and php much) the community. If you run into problems, how likely is the modx vs textpattern forum.community willing to help. A good indication is community size. While I can't see the numbers for textpattern, currently modx forums show "255 Guests, 46 Users (3 Hidden)"
good luck