Well, it depends if you want the best action or the lightest weight. I can definitely relate, recalling injuries of the past lugging around my old Hammond M3 (the one with the full hardwood cabinet).
What type of keys do you play? Do you want weighted-action, for playing realistic piano patches, or are you playing more synth and organ (which tends to want lighter action)?
My two primary controllers provide the different action, considerable weight difference. The Roland A-80 is, in my opinion, the best damned weighted-action MIDI controller ever built for realistic piano action and live action features (and it's 88 keys). However, it's bulky and heavy (I own 2 of them). They've been out of production for years, and professional studios have most of them. I picked up my second one from CraigsList recently for $425.
The M-Audio line has quite a few options, pretty much all light in weight, and in different sizes. The action is nowhere near piano, though, like the A-80.
In my view, it's hard to get really good weighted-action without the weight. So there needs to be a decision made as to what the priority is.
I hope this helps!