Why so many trolls here?
The Lexus IS is considered a "compact" and Acura TLX is considered "midsize" class. It is best to test drive and see each vehicle in person to make up your own mind. Sales people will try to push for a sale. Always sleep on a decision before making one.
A note though: the IS is RWD (or RWD bias in AWD models) and the TLX is FWD (or FWD bias in AWD models). The car handling between the two drivetrain is completely different.