In that case, I'd say the TK exchange is pretty good value.

I generally don't suggest that people spend too much energy worrying about getting optimal value out of their miles / points, because they're rapidly devaluing anyway.

That said, how many miles is the equivalent UA redemption? UA gets an extra 10% on Marriott redemptions, doesn't charge $500 in fees, and will give you PQP toward status requalification for award flights on its metal. And, if something goes sideways, you're dealing as a status member with UA rather than as a non-status member with TK.