Windwalker,

These two engines are not on the same turf.
The MVVS (with a silencer) costs almost twice as much.

Below 12,200 RPM, there are not many engines that can spin large props as fast as the MVVS. The recommended prop sizes for it include the 14x7 (and the 15x5, about equal in load). It is timed and tuned for large props and low RPM, in the interest of lower noise levels; especially with the full length tune pipe, or the tuned silencer.

In the OS web site, there is a review for the .61FX and Clarence Lee did a review in RCM on the MVVS .61, of which the MVVS .77 (which he also tested) is a stroked and bored version.
The MVVS had a 2,000 RPM advantage over the equal displacement OS, on the 14x6 prop size.

So you can see the clear bias toward lower RPM levels.
The bias is greater in the larger displacements. The MVVS .77 and .61 provide equal power from about 11,500 RPM.
At lower RPM, the .77 excels.

With regards to the Tower .75; the .61 proved to be the strongest low-cost .61 around, in the shootout by Dave Gierke, in MAN 05/2003.
If the .77 is anything like it, it will be likely to out-rev the MVVS on smaller prop sizes; 12x6/11x8 and less, which are not at all in the usable size range, for the MVVS.