RE: Yak 54 vs 55
None of them is "better"? That's going to be your call. Some stuff I might be looking at trying to make that call for myself?
A pattern plane might be thought of as a more stable platform than something more suited to the tumbling 3D types. One of the ways to enhance stability is through the use of a longer tail moment?
3D types will have bigger control surfaces that flutter more easily than pattern types that are generally run at higher speeds. That's not to say the 3D type surfaces can't be cut down or rebuilt a little smaller?
Power vs. weight vs. wing area? Those 3 factors all play together, big time, to predict a plane's behavior as well. Higher wing loading for better wind penetration and stability, less wing loading for more predictable low speed, possibly high alpha control.
Frontal area/drag to help control speed on down lines? Big radial cowls and large radius leading edges much different than streamlined w/sharper leading edges?
Just some ideas. My preferences here may have nothing to do with what you think might be right for you/what it is you're trying to accomplish? Best of luck on your choices!