Using Sheldon Brown's GI calculator to compare bottom gear:
39 42
41.0 44.2 (25T)
36.6 39.4 (28T)
30.1 32.5 (34T)
The OP will determine if the 39T/28T is satisfactory. The ~32% decrease in GIs using the 39T/34T from his OEM 42T/25T is hard to ignore tho'. The 34T will require a mountain bike RD and a new chain (longer) and will be about as good as it can be. My sister has a mega range cassette and while I find the upshift from the 34T quite abrupt, it's a good bail out gear.
Brad
Last edited by bradtx; 01-21-14 at 12:32 AM.