There's something to be said in favour of a very pure digital echo.
Much of the appeal of the old tape loops is that they introduce tiny variations in pitch which, when overlaid with notes of a similar pitch introduce - depending on the amount of difference - chorusing, beats and phasing (which is just very slow beats), modulating the amplitude of the note and making variations in the timbre. This gives the impression of a richer sound, more vibrant, more organic.
Tiny variations in pitch are inherent to theremins already. Just holding a note over a delay automatically introduces the above effects, because you do not stand perfectly still. These effects are most emphasised when the delayed sound is not degraded, when it retains all the harmonics of the original tone, so that they can all interact with their approximate copies. This is done best with a straight digital delay - the higher quality the better.
From this point of view, the things to look for in a delay is the quality of the conversion to and from digital - I have an old Boss PS2 Pitch Shifter that doubles as a delay and it is soooo harsh - the pitch shifter I like, but as an echo box it's not for me. Then I have a Marshall Echohead, which is just silky on the pure digital setting. It also has some perfectly fine settings that give it a good old-school feel and I hardly ever use them - IMO the timbral richness that comes from my hand movements is unique to me, part of [i]my[/i] sound, and hence preferable to a programmed emulation of a mechanical imperfection that we have grown attached to for mostly sentimental reasons.
(I would also look for how short a delay the device can achieve, and if the delay allows true 100% feedback. The continuously variable pitch of the theremin is great for exploring the resonant frequencies of a delay on the order of a few milliseconds. At these durations an echo box is a feedback comb filter (http://en.wikipedia.org/wiki/Comb_filter).)
