They are both good weapons. Depends on your playstyle. I have an aggressive playstyle so i favor the katanas. Good damage, bleed build up, fast strikes. Your able to strike and get out to whittle down health or go all in 2h and mash that r1. I tend to use the bss more defensively. 1h and 2h pokes from distance, looking for counters hits. i use the 1h moveset for unlocked play. I tend to get in and out with this weapon, i rarely go toe to toe againt someone with it. Both are viable and deadly, play with both and see which suits your playstyle best.
I like the side sword better because it's elegant, and the r2 is fast /strong enough to be useful unlocked. But it's fairly useless against turtles with a high stability shield because it won't break their guard; you need a shotel or something on swap for that. An uchi can still get a bleed and break their guard that way.
smash the control images, smash the control machine.
The Uchi has more going for it overall and is undeniably better in PvE due to bleed. Both can be buffed, which makes the otherwise terrible 1h R1 string on the BSS temporarily quite decent for those 60 seconds. A buffed Uchi is especially annoying to avoid if your opponent starts chucking out unlocked 2h WR R1s (DWGR of course). Spears are off course pressured by all Kats WD R1's due to their high damage and good lateral range, meaning the Uchi has a better match up against the dreaded Long Spear setups, not to mention, again, unlocked 2h WR R1's.