I repeat :), "if you had to pick only 2"... "current devices to test on"... "Roku1 + Roku4 sounds like a good (maybe the best) combination".
Given strict limit of two devices. Bracketing it with a Roku1 on the low end + MIPS cpu + DirectFB graphics. Bracketing with a Roku4 on the high end + ARM cpu + GLES graphics. Then things in between lean to one of the two sides - RokuTV towards Roku1, stick/Roku2/Roku3 towards Roku4.
If one were to write app that runs well on both of these, i am fairly certain it will run well on everything in between. One caveat - i generously assume they won't be checking model number like in viewtopic.php?f=34&t=94798