Difference between a 20 and a 30 is that the 30 can speak to some[1] Ant+ sensors (chiefly heart-rate monitors); that it has a barometric altimeter to reduce the error inherent in reading elevation from GPS; and a magnetic compass so the map/arrow always points the right way[2], even when you're not moving.
I expect the difference between a 25 and a 35 is similar. IIRC the 'x' models are just a minor improvement to the screen.
The little joystick thing is often infuriating (particularly for text input) if you're used to modern touchscreen devices, but it has the substantial advantage that it can be operated while wearing thick gloves.
All eTrex units are excellent at recording a track of where you've been - unlike the cycling-specific Edge units, they'll quietly get on with it in the background at all times, rather than expecting you to press a button to start and stop the recording, so you don't end up with the problem of it not recording your ride because you've forgotten to press something. They're less good for racing/training analysis for the same reason.
The navigation features are broadly similar, and work best if you're prepared to put a bit of time and effort in, both learning how best to use it to suit your style of navigation (do you want it to tell you what to do, display a line to follow, or just be a waterproof backlit map with a 'you are here' arrow that you never have to turn over?), and at your computer creating the relevant routes/tracks to upload to the unit before you set off.
Anything AA-powered is a no-brainer for touring. Expect to get 2-3 days of use from a decent set of NiMH batteries, slightly less from alkalines.
The handlebar mounts for the modern eTrex units are often referred to by retailers as the Oregon/Dakota mounts, as they fit those units as well. It's not as compact as the Edge series brackets, but it's designed so that the back of the unit is more comfortable in your hand, without sticky-outy bits to click into the mount. They work well, although the cheap eBay copies can sometimes be too loose.
[1] Conspicuous omissions (compared to the Edge) are wheel speed sensors[3], and cadence sensors that aren't combined with a speed sensor. Yes, this is ridiculous.
[2] This doesn't work very well when it's attached to a bike on account of the metal, but it's a welcome improvement if you use it for walking.
[3] Much like having a barometric altimeter, the logic behind this is that it gives a more accurate measurement of speed/distance than GPS alone.