Quite a bit lighter, Kev. Taking my figures from the ever-reliable Digital Picture -

Manufacturer specified weight. 500/4: 3.87kg; 150-600: 2.86kg. (Excludes hood and, in the case of the Canon, the tripod foot.)
Actual weight: 500/4: 4.015kg; 150-600: 2.86kg. (Still excluding hood.)
In-use weight: 500/4: 4.26kg; 150-600: 3.155kg. (A 35% difference.)

These figures should not be surprising. Both lenses are very solidly built for heavy-duty use with no shortcuts, but the 500 gathers roughly twice as much light and consequently needs more glass. It's actually surprising how light it is. (And the weight of the new Mark II 500 - 3.42kg in-use - is a downright miracle!) My guess is that Sigma were very keen to position the 150-600 Sport as the serious, professional quality lens it is, and were happy to spend a bit of extra metal on it to be sure that no-one doubted its build quality. It is interesting that the cheaper version of it - admittedly with different glass - comes in at just 2.035kg in-use, or less than half the weight of my 500. Even more interesting is that both the Tamrons weigh just 2.11kg.)

All that said, in my brief experience with the Sigma, I found it rather more difficult to hand-hold than I expected. (Ditto the Canon 200-400/4.) Was this (a) something to do with the weight distribution, I wondered? Or was it (b) to do with the more restricted choice of gripping points as the two zooms include both focus and zoom rings where the prime only needs a focus ring? Or (c) simply that I've owned the 500 for many years and am not used to holding the zooms? I'm inclined to suspect that (c) is the most important factor, but not the only one.

Anyway, I found the 150-600 surprisingly awkward, but I'd be happy to revise my opinion in the light of more experience with it. Feel free to post it to me and I'll try it out properly for a year or two and report my findings.

^ All that aside, my experience with the 500 was that I was aghast at the weight of it on first impression and didn't think it was anywhere near hand-holdable, but quickly became accustomed to it and have hand-held it regularly ever since. I reckon you might find the Sigma similar. Give it time and see what happens.