Also do you think riding a recumbent makes a difference over which you choose? As in would you still go Oregon for an upright?
There's recumbents and there's recumbents: With tiller steering, it's no different to an upright. With under-seat or open cockpit steering you tend to have to get creative. With many trikes you have the option of mounting it off to one side, which put it off your eye-line, but within easy reach. With most USS bikes, you end up having to mount it on the dérailleur post or a clever headset bracket for people without sturdy thighs. At which point a larger screen becomes desirable, but only to the point of not adding too much width or height (you have to get a leg over it to mount the bike, and GPS brackets don't like being kicked on a regular basis).
The real issue with boom mounting isn't one of size, so much as user interface. You pretty much need to stop to operate the GPS, which means it has to get on with whatever you want it to do with minimal user interaction, and you can forget about making out fine or colour-based details on the map screen. I use my eTrex in auto routing mode with City Navigator maps for this reason, as the turn-by-turn popups are clear and appropriately zoomed for each junction. Following a breadcrumb trail Wowbagger-style (which I'm happy to do on an upright bike) would require stopping and sitting forward to examine the screen at non-trivial junctions. If I'm going to do that, I might as well use a map.
If I were starting again, I might consider something with a significantly bigger screen, though I wouldn't want anything too chunky, as it still has to play well with lights on upright bike handlebars, and I can see a tiller steering 'bent in my future. I'll be interested to see if the map rendering on the new eTrexes makes a significant difference.