Unfortunately it's not really possible to play with the same list at both scales, due to things missing in one scale or the other, or price of things in different scales, or just how they work in game. That said, I've collected enough alternate units and alternate sculpts that eventually my full Warhammer army will be in Warmaster scale. I was, for a while, modifying my Warhammer army to suit the Warmaster one as well- making the orcs 5 wide and 6 deep (Warhammer orcs need to be modelled as a unit- they are too large for their bases). Unfortunately I drifted away from Warhammer with 8th edition, so that army hasn't been touched in ages. From current rumours 9th ed sounds even more awful.
For army composition between the two scales, I prefer the true 'horde' feel of Warmaster, though miss having lone rock lobbers and doom divers. I'll have these in 2000pts of Warmaster Orcs, but it won't quite be the same. In both systems cavalry are a bit too powerful (and therefore common), though I purposefully steer clear of taking too many now. 8th ed Fantasy did a good job of reducing cavalry effectiveness, but too much IMO- now cavalry do very little in that game.
As for how they play- unfortunately I've actually only ever played one game of Warmaster. I hope to play more in this year though! One similarity that struck me is that wolf riders die horribly in both systems, achieving very little. Goblins on foot aren't much better.
I think animosity will be much less of a problem in Warmaster (I take this to be represented by the low Ld characters). In Warhammer, animosity has always been my bane. It's supposed to be a 'double edged sword', and 'mitigated by having many more units', but in practice it just doesn't work. Units failing to move is annoying, and charging out of formation unsupported is even worse! And though you may have more units, the physical restraints of space mean you simply can't have all units properly positioned to support each other in the event of poor animosity rolls (even if you played on a table with no terrain). This is even worse since 8th ed, with hordes and units expected to be even more massive.
Despite the difficulties, I still like my Warhammer Orcs. They lose a lot more than they win (if I want to win I have a Wood Elf army), but it's usually fun to play- and sometimes they'll really surprise you, pulling off very unexpected victories.
