155 cm ~ 61 in ~ 5 ft 1 in
I think a Medium is way too big for you, probably more like a Small (I ride a Litespeed compact sm).

Note that Merlin generally has longer top tube lengths than Litespeed. E.g., some people who can't fit a Merlin Agilis can fit a Litespeed Siena because of this, so take that into account.

Some women have short legs or short torso; others are just smaller (proportionally to a man). Thus, don't just consider a WSD-type of frame.

Given your size, I would choose Ti or Carbon. If I got Al, I'd make sure it had Carbon stays on it, but I'd probably just avoid Al altogether. A good Carbon bike you might consider is the Giant TCR Composite series. It may be in your price range as well.

Generally, Litespeed and Merlin try to dump their inventory around Sep so you may find some good prices then (I bought mine in Sep of 02 for ~35% off MSRP.

Whatever you do, try to spend your money [a] frame-fork [b] wheelset [c] components in THAT order. E.g., Bike A has a superior frame, better wheels with a 105-Ultegra build and Bike B has a lesser frame but full Ultegra build. Assuming you can choose between either, "A" is a no-brainer decision.

Best.