Hmmmm... this is a tough one. I own both, and if I had to choose for hard rock / metal, I'd probably take the Soldano.
To me, the Helios' strong suit is 70's mode for plexi tones up to un-boosted 2203 type sounds. I'm not crazy about 80's mode with the diodes. It's a little compressed sounding to me. Don't get me wrong though, the Helios is a VERY nice amp, but it's the "metal" part of your question that's bugging me.
I feel like the Soldano Hot Rod 25 lead channel starts at a hot 2203 type tone and just gets heavier from there. Might be better for what you're after.
I do agree that the Helios sounds a little more raw than the HR25.
One thing that's really special about the HR25, and I don't know how Mike did it considering the amp is still running a pair of 6L6's, but you can open up the master to get whole the amp just humming, and the volume is not killing animals and small children. It really is a 25watt amp, but it sounds HUGE!! Pretty amazing!!
Any reason your limiting your search for a hard rock / metal amp to just these two?