this is a more complicated answer
I own a Dingwall Combustion (37" Multiscale), a Yamaha BBP35 (34" scale) and a MTD Kingston ZX (35" scale), so I think I'm pretty qualified to discuss.
Ignoring the differences inherrent in tone from electronics
The upsides of multiscale:
- The Dingwall's low B String hits harder than any other Bass on earth. It is tight, it is clear. Not unlike a Grand Piano.
- The volume balance and feel between strings is very consistent.
- Increased sustain is pronounced, a shorter scale instrument isn't physically capable of competing.
Downsides
- Above the 12th fret the multiscale fret placement can be difficult to deal with, especially playing chords. Scales can be a little goofy as the fret spacing difference between strings is odd. If you don't play up there, it's not an issue.
- The neck on the Dingwall is LONG - 3" difference between it and a traditional 34" 'Fender neck'. If you play extended scale runs this can mean some pretty good stretching of the fingers. It's a long ways from the 7th fret E string to the 9th fret G string
Worth noting
- The Long scale basses sound like modern basses. If you want that short scale thump like on a Beatles record, it's not really there.
- Likewise for metal or prog, 34" just won't give you the same clarity as the multiscale.
- I spend very little time on the open B string in gig playing. The 5-string's main appeal to me is to be able to play across the neck. IE - the 8th fret G on the B-string allows me to run 3 octaves without changing hand position. For this, the multiscale doesn't really have an advantage anymore.
- My main Bass for my current project is the 34" Yamaha, but again that's because I don't spend much time below E1. This is a more class sounding band. If I play metal or prog stuff, the Dingwall comes out
What I wouldn't do
Buy a multiscale that
isn't on the 37" B. You get all of the disadvantages of the multiscale neck, without the advantages. The 37" scale length isn't arbitrary, it's the correct speaking lenght for that note/string size.
Running a multiscale on a 35" B - is (IMHO) stupid. You don't get the power on the low strings and the scale length on the high strings gets too short.
If you aren't going full 37" MS, just buy a 35" linear scale bass.