I understand BAT runs the VSM 6.0 sound mod. Do you have to manually update to VSM7.5 plus the Merlin enhancements [...] ?
If memory serves, yes. Maybe check the 4.3 changelogs again, but I don't think BAT has VSM 7.5 yet, and I'm pretty sure it hasn't adopted the Merlin enhancements.
Does the Next Generation sound mod by FL2070 offer an alternative set or is it down to personal choice as to which to use.
Yes, the two mods use completely different sets of samples and have completely different design philosophies resulting in completely different experiences.
For those wanting the full explanation:
The difference between the two comes down to stability and content versus new features.
VSM is built on the stock game's sound code, and has been in development for a few years shy of a decade; Victor has spent many long years adding tons of content to VSM and endlessly tweaking everything to perfection which offers great sound variety and rock-solid consistency. VSM has something like 20 or 30 more different sets of engine sounds across different plane types than NG-USM. It's also specially made for BAT, and features a full sound replacement for all eras from DOF to JTW, while NG-USM does not offer anything for the DOF module.
NG-USM, on the other hand, is built on the expanded sound code provided by the mod "Next-Generation Sound Engine" which is a mod that massively expands what sound mods are capable of doing and also the ability of users to customize those features. The other side of this is that it is a less matured mod that is less stable with less content variety. Users report vastly different and sometimes even directly contradictory results.
If you want a tried, tested, and true one-stop-shop to instantly upgrade your sounds to a world-class reliable standard, get VSM.
If you want to try out the new features, get NG-USM, but make sure you check your logs after running the game, and be prepared to tweak some configuration files a bit.
I'd recommend anyone who cares enough about the game's sound to try out both options to see which one they prefer.