Short answer: I have always bought flights through the travel portal rather than transfer UR to airline partners. Long answer:

When you transfer UR points to partners, the question is what redemption value do you get for those points in the partner's system? Frequently I am finding the points are worth just $0.01 each. For a specific, immediate redemption the value you see may be higher or lower.

When using UR points in the travel portal, the value per point is fixed: $0.01, or $0.0125 or $0.015, depending on the multiplier you get from your best card. You've got to check, though, that the prices shown in the travel portal match the best prices you can get on the same tickets elsewhere. Often they match up but sometimes the portal costs more. Buyer beware! But bottom line, with a CSR card and the $0.015 redemption value it provides, I've always found the portal a better deal than airline transfers.