Just a matter of preference. Having a fixed wheel
gives you a bit more control rather than just pedaling
to go and braking to slow down--this is if you are comfortable
controlling your bike directly through the drive train and it
can be offset if you don't run brakes and aren't good at
skipping and skidding. Lots of roadies ride fixed
conversions for training and such. Have you considered
buildign a conversion.
If you want to be able to go back and forth
try the Kona Paddy Wagon--budget priced, fixed
and freewheel come stock, has two brakes
for when you are running a freewheel. Looks decent overall.
Get someone to show you what proper chain tension
is and how to adjust it. Don't get your fingers caught
in the drive train.