Heh. You know asking people with hundreds of thousands of points worth of models "which ones should I keep?" might not result in the most productive answers...
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Personally, I'd keep 2 companies of SHTs as-is and use the rest for conversions to other SHTs, as several people have suggested.
The arty kind of depends on the actual models. I'd keep a company of each and a bombard battery or two (maybe a company of Bombards if you wanted to play them as a company in a friendly game).
The Hydras might be a bit much, but I'd definitely keep 8-10 for larger games.
Aside from that I'd keep it all, and since the above suggestions don't really suggest getting rid of any of it... that's pretty much "Keep it all."
Like I said... maybe not your optimal audience.
