I would vote for the Ryobi if not for the rear carb, which make hooking up throttle and choke cable somewhat more challenging. Weedwackers, in general, are more difficult to extract the engine - often requiring reading some instructions if you haven't done one already.
Most of the weedies require some cutting and grinding. Even the Ryobi requires some "machining" of the rear plastic back cover. Last, the weedie carbs are pretty weezy and need to be replaced or modified to make good power.

My vote goes to the Poulan chainsaw engines. Extracting the engine is simple and straightforward. Once the engine falls out of the case, it is ready to eat. No cutting. Plus, you have a wide range of displacement to choose from - 33cc to 46cc. If you are going CH Ignition, get the 46cc. If you're staying magneto, the 33cc to 42cc ones work better. The OEM carb give good power and can be used as is, and many have butterfly choke valve built-on. Mounting is easy and more intuitive for folks who are used to beam mounts RC-style.

Homelite chainsaw are reported similar to the Poulans in ease of conversion, though I have no personal experience.

Just my $0.02. Actually, if I had put my $0.02 every time I played with a new conversion engine, by now I probably have enough to go to MacD to buy an icescream cone.