I think step up is better than step down due to vignetting (prevention thereof). But bigger filters cost more than smaller ones... if the difference is not too great, then get the filter in the bigger size, and adapter for the other lens. I've done this myself, I have 52, 55 and 62mm filter size lenses. And I'm standardising on 62mm filters, with cheap adapter rings on the others.
Handy hint, if you plan on keeping the step up ring on the lens to save time, then get a matching size lens cap as the original wont fit any more.
_________________Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.