Winternitz One time signature security
W-OTS+ is stronger, as it makes weaker assumptions on the hash function.

Let us take a rather extreme example, let us consider W-OTS and W-OTS+ based on the MD5 hash function.

Now, the proof for W-OTS is quite invalid; it assumes that the hash function is collision resistant, and we know how to generate collisions with MD5.

On the other hand, W-OTS+ based on MD5 would appear to be secure; despite MD5 being "broken", we do not know how to create second preimages to MD5.

BTW: this appears to be more about the proof technique used to prove W-OTS, rather than the actual security; we can create MD5 collisions (invalidating the proof), however we don't know how to use those collisions to actually attack W-OTS-MD5; hence it would appear to be secure (but we can't prove it).

Also, one note: those aren't the only versions of Winternitz signatures in the published literature...