It depends on what your intended use for the fiver will be, so you'll need to answer that question first. Generally for solo classical work, one uses a 4-string with solo strings (tuned one step up form the standard bass). The literature and most of the standard fingerings assume this setup and will give you the most options for solo pieces. Additionally, for solo work a smaller body bass works better because it will have a better sound in the higher registers. Generally a fiver has a bigger body to project the low B or C tones and are not as well suited for the higher registers. Don't get me wrong, a fiver with a high C can be a lot of fun and give you access to the higher registers more easily than a 4-string, but it is more of a novelty and probably not as much in demand from a resale perspective.
I bought an old (somewhere between the 1880's and 1920's) German fiver last year and have really been enjoying it. It is set up with a low B and is a pretty large, violin cornered body. If you are used to playing a 4-string it takes some time to get used to it, but once I did, it has been a real kick. If you plan on playing the classical literature in a symphony orchestra, you will be suprised how often the bass part calls for notes below the low E string. It's usually only occurs in some isolated spots, but when you play those notes in the correct octave it sounds great and really puts the bottom to the chord. I am at the point now where when I play my 4-string in the orchestra it feels like I don't have complete bass! It's great to be able to play those low notes easily without messing with an extension or reverting to playing them up an octave.
My advise is that unless you want a very specialized instrument, go with the fiver with a low B or C. If you want a bass for solo work, get a 4-string and set it up for solo playing.
Tom