Many things still need answered, but either mower will work. It is just a 5' has pros and cons as does the 6' for your tractor. Which "pros" apply depend on how you answer the following....
What are you cutting? How dense? Any heavy stuff? how often?
Are you mowing in a wide open field? or is manuverability a concern?
How heavy duty of a cutter do you want?
IMO, in 90% of circumstances, there will be no "finished" time difference between the 5 and the 6. Sure the 6 cuts a wider swath, but you have to go slower. End result is usually the same.
So other factors come into play that are far more important.
If manuverability (or total tractor weight and length for trailering or soft conditions) is a concern, then get the 5'-er. Especially if you want a heavy duty cutter. These kubotas are pretty light for their HP, and I have both a med-duty 5' cutter and a HD 6' cutter. The 6' cutter requires I leave the loader on. Thus it is more difficult to trailer and MUCH less manuaverable. The 5' cutter is narrower (or at least the same width) as the tractor, lighter and tucked in closer, so I can talke the loader off. This is nice for tight areas and trails in the woods.
Heavy thick slow cutting stuff, you have to drop down a gear with the 6' cutter. Still takes the same amount of time overall cause its cutting a wider path, but it just "seems" sooo slow.
So my reccomendations is....If you are cutting a wide open area, that is easy cutting, and you cut it several times a year to maintain it, you will be happy with a 6' cutter.
BUT...If you are in tight quarters, or cutting thick nasty stuff, or plan on trailering much, I'd go with a heavier duty 5' cutter.