Try them both - see which you like better. The roundwounds will give a brighter sound, the half-round will not eat up the frets, fretboard (and your fingers) as quickly.
I run D'Addario half-rounds on the Gibson RD Artist 4-string (originally, I used RotoSound rounds), and RotoSound rounds on the Epiphone Les Paul 5-string. Half-rounds are not as easy to locate, especially in 5-string.
Some of the sound will also depend on the setup on the bass.
Jim