Lithium BMS vs. Manual battery balancing
Opinions...which of the two is better and why?
By manual I mean...take out the cells and individually, say, charge/top-balance them off individual chargers yourself.
Or does it depend on which BMS and which ones would you recommend and why?
There is another option: install balance wires on the cells (which you have to do for a BMS anyway, IIRC) but no BMS, so that you can check and charge them periodically with the bike unopened. My 2009 VX-1 has a set of balance leads, installed by Kocho (calling Kocho!) when he converted it, that terminate under the seat. The main objection to having a BMS is that it's one more piece of complex tech that can fail, possibly taking the battery pack with it. The main advantage to having a BMS is that it will protect the pack from being run too low, overcharged, or getting way out of balance. (Some basic BMS units don't balance cells, just monitor the pack for under or over-charge.) Checking the voltage of each cell on your bike may help to make the decision: if they are already out of balance, that would suggest that a BMS would help. If they are all pretty close together in voltage, that may argue for just the leads...
You are, of course, going to want to know "In the case of Vectrix Leaf conversions, which is the better bet, a BMS or just balance leads?" The problem is that Leaf conversions are so new, and so rare, that the jury is still out. Even one BMS failure would be a significant minus (I don't know of any, but again, I'm no expert here), while no failures would just mean that it isn't a huge issue...at this time. That's why I suggested using the state of balance of your pack right now as a factor in deciding. It's a classic cost/benefit analysis, with the benefits well known but the costs/risks not yet established.