Hmmm, never tried the Savage 120 (here in Australia, its a special order just to get one in), so I can't comment on that, but I am the proud owner of an Invader 150 for around 3 months now, and I must say, that I am delighted with what that amp can produce.
My tastes are very picky when it comes to overall saturation of an amp, as I love very thick but straight to the point kinda distortion. I love my 5150, 6505+ and my XXX, but the Invader is another couple of pegs up. Everyone that has heard this thing when we jam, are blown away by how much better it sounds than the Peaveys.
You say you boost the Invader? I boost all the Peaveys with a Maxon 808 and they sound great, also tried it with the Invader, and I swear it made it worse, even trying different settings on the amp itself and also the Maxon. Have you tried dicking around without a boost?
The 4 channels are very different, with the cleans being great, channel 2 is like hot rodded Marshall territory, channel 3 is the most scooped and brutal sounding, with channel 4 being awesome for lead tones.
Im using a Presonus EQ-3B in front of the amp and that has tightened things up brilliantly. As I said before. The 5150 is a great amp, and I love it, especially for it's mid-range grind, but the Invader does better book ends, as in the lows are deeper and tighter and the highs have a better quality sheen to them. Mixed with the Peaveys, I should have the sound I've been looking for. I have only tried all amps through V/30s, so I can't comment on other cabs, and my musical tastes are more death metal like, rather than this new-age metal kinda thing, so I hope you can get a bit of a feel from where I'm coming from.
Would love to try a Savage out, (Exodus' newy is the best tone I have heard for a thrash metal album), but sadly here in Oz, we only get Powerballs and Fireballs, Blackmores and Gigmasters. I tried a Powerball 2 the other day through an Orange cab with V/30s and walked away thinking that I could achieve 99% of what the Powerball had to offer through the Invader, so no moneys were parted. The Invader is also one loud mutherfucker, and because of its mids, cuts through brilliantly well in a live environment also.
I will get a Savage one day (thomann Germany), 230v works well straight into a wall socket here and at 60% off of what we get stung here for an Engl and I'll pop back one day and do a comparison of both Engls. All the best with the decision.