Well heres an idea, keep on doing 5 x 5 with the 20kg untill the last rep becomes possible, then up the reps to 6 x 5, then when your ready and up for it, do the same with the 25kg (keep on doing it until it becomes possible, then go up to 6 reps, then when your ready, go up a weight).
Good luck!
Edit://
your theory on seeming to lift less if you do 6 x 5 isn't really true. I mean, your not lifting 500kg are you? Your lifting 20kg, just loads of times. I understand your thinking though, but it don't work like that. If your doing 6 x 5, then you'll be doing 30 reps of 20kg. Not 30 reps of 500kg lol. Get me?
Sorry i've got the wrong end of the stick, but thats how your last post came across to me
