for the same TX Power (for example, 2W), the lowest frequency will have the strongest Rx signal.
The reason is that the lower frequency propagates better. In Wikipedia, look for “free space loss”.
Your second question is uncorrelated to the 1st one, I believe. The MS doesn’t display bars depending on the RxLevel, but rather depending on the criterion C1, which also depends on parameters such as RX LEV ACCESS MIN and MS TX PWR MAX CCH