AAs mean more faff at home when you charge the batteries at the end of the week or before the big event. Internal non-replaceable batteries mean more faff sorting out USB power during your long bike ride or tour. Both are eminently manageable, though I note the relative faff-factor of AAs has gone up for me as less of my other kit uses them. In general, I prefer to do my faffing when I'm not tired and stupid and/or being rained on, which is a win for AAs.
One note on breadcrumb trails: Ideally, you still want a map for the trail to be displayed over, rather than simply a line on a blank screen (as you'd get on a traditional hiking GPS like the eTrex 10). Otherwise roundabouts and similar complex junctions get reduced to educated guesswork.
The main difference between the eTrex 20 and the 30 is the barometric altimeter (for more accurate recording of elevation, particularly in mountainous areas where GPS performs poorly) and the ANT+ communications (for heartrate, temperature and some cadence sensors, directly exchanging data with similarly equipped Garmins.[1]).
[1] I've owned mine for years, and finally got to use this feature at the YACF camping weekend in September. Astoundingly, it Just Worked™.