Your neck bow is contingent on your playing style
and the bass itself. Strings vibrate eliptically, hence the usual up bow of the neck to provide for some relief for the string to vibrate. If you take your friends bass and hold the string down at the first fret,(by the nut) and the last fret( by the bridge), I bet you will see some space between the string and the frets
somewhere between the 5TH and 10Th frets(usually greatest at 7 and 9). I would doubt that many bassists play with an absolutely flat neck. And I would also argue that you can probably get a lower action with some relief, than with a flat neck. IE: If your neck is flat and you're getting a little string buzz you will have to raise your action to eliminate the buzz making your action higher.
So it really isn't a question of which is better,or best, but of which is better or best for you and your style, technique. The bass will also dictate how you set it up for optimum performance according to your playing.
I guess this is a long winded vote for:
A LITTLE BOW