That was one of my thoughts - it's more powerful than lab early, but weaker later on when you want a lot less to actually trash stuff. It should be an extremely strong engine enabler, I guess.
The only possible argument for pricing it less than 5 would be the same one as Chapel costing what it does - that it needs to be priced so anyone can afford it, if there's a possibility of it being afforded by someone round 1. By which logic it's best priced either at 4 or 6. And I think the trash-then-draw variant is the more balanced one.
I like the idea though, enough that I might actually want to playtest it. Shame my gaming group doesn't like dominion as much as I do D:
Unless I'm missing something both of those make it better. Making the trashing optional is strictly better (no one bring up possession ok). For the cards in the OP, the first sounds too strong at $5, but could be balanced at 6, though I think not that interesting. The second seems way too strong.
'if you don't, discard two' make it worse than warehouse if you choose not to trash... Difficult to write it concisely though if you want the trashing first and the discarding after...