I've had MQs and Riff Raffs in the same guitar - two guitars actually. For what I do, they're quite similar in what they give me.
In a hard sounding and bright Epi SG, I found the MQs were too much for me - the Riff Raffs were better.
In very warm and open sounding Gibson Explorer, the Riff Raffs were doing OK but weren't quite cutting through. When I put the MQs into it, that was it for me. It makes what I'd always imagined was "PAF" noises (I'm not sure they are really though :lol:). It can be creamy, it sings, it bites, it responds to fingers and volume/tone controls.
The Riff Raffs are now in my new SG, and I was doing some ABing the other night for pickup height adjustment purposes (actually ABCing - the Mule'd Tokai Love Rock was involved as well) - on the amp models I was trying out, the MQs in the Explorer have the highest output out of the three, it pushed the amps a fair bit more than the Mules or Riff Raffs.
I'm not sure what I'd put in a semi myself, but I would lean quite close to MQs I think. They give me very close to humbucker tone, and then with a slight turn of the volume, or lighter picking, they clean up and get all "jazzy" and bell-like on me... In the right guitar they're kind of the best of all worlds for me.
What would decide me would be what the accoustic tone was like - if it was a nice warm woody tone I'd be onto MQs immediately. If it was a harder, edgier tone, I'd be a little worried that they could be harsh for me - they are a very aggressive sounding pickup through the amp settings I use :lol:
Something to consider though - and Fourth Feline has found this as well (in a semi) - the MQs are really responsive to pickup height adjustment. You can get them fat and powerful nearer the strings, clearer and slightly less powerful further away. My Blackguards almost respond that much, but my other BKPs seem to give more subtle changes, very important changes, but less noticable than the MQs for me.
Lastly, how much do you want that "bridge humbucker crunch" for chugging chords and rhythm work? For me, the MQ gets me close enough. But I do agree with Philly - it's not quite the same with a P90, it's slightly less compressed and focussed than a humbucker.
EDIT: Just been reading something Wez posted elsewhere, what strings do you use? I use 11s - it's just crossed my mind that I might get a big kick from my MQs because of that: heavy strings and quite an agressive picking action. It's possible that the tamer version of MQs, Manhattans, might be a way to go if you're the same? (But on the other hand, you've got Blackguards, I've heard your clips, MQs are probably right up your street! :lol:)