Generally speaking I'd prefer single sided for the reasons mentioned earlier.
Also, a lot of white space'll make your material much friendlier and easier to read.
Also, if you prepare stuff double sided, I can guarantee -from bitter experience, of course

- that sooner or later you'll need to run off some extra copies, and your learners won't be too impressed to discover they only have pages 1,3,5 .... cos you forget to press the right button on the copier !
If it's going to be fairly bulky should you maybe send copies out afterwards to save people lugging them around on the tube etc. ?
What kind of material are we talking about here?