depending on your priority...but IMO..better topup a bit and buy the 925 since you no need to update this and that..

unless you do not prefer a 16GB storage capacity

if you prefer 32GB for the 920

designwise..i think 920 win hands down here..

by update i mean the latest 925 get the latest update preinstalled without you ever need to update it
save some precious times there..

meanwhile in 920 you have to update all by yourself because older stock do not come preinstalled with amber update
IINM..unless you get a newer stock which might come with the Amber update preinstalled

but you can always ask the staff at nokia centre to install the update for you

PS: all lumia 9xx series will surely receive future update so no worry here.