A very good question, and a very interesting comparison, so I humbly add my few words to it.
BA10 and MT300 are very different for an easy simple comparison, so trying to break it into different facets.
Technically:
-- Tuning - while MT300 are surprisingly coherent for a tribrid, my slight preference is with BA10, still, arguably, the best tuned KZ.
-- Resolution - a bit hard to call at first: MT300 definitely make a more resolving impression, but how much of it is the EST sparkle? (akin to those type II tapes). Then, thinking about it, still MT300 overall, the strings can sound mighty nice.
-- Bass - definitely a matter of taste: nicely implemented BA bass in BA10 (I would not much agree about DD-like, that would be Sonion BAs to me, which I personally dislike); MT300 has one of tge best DD bass for me - resolving, not intrusive but present and well integrated with the rest.
-- Mids - the weakest part of MT300 to me, one BA covers mids reasonably competently, but if you compare with Knowles ED29689 or even C16 and ZAX, mids of MT300 are less to me. BA10 mids are a bit more rounded in this aspect, but still mids were not something KZ thought of as a tuning priority. The graininess of old Bellsings is also the most apparent in mids.
-- Treble - MT300 definitely wins EST (and BA) do the nice job; BA10 have quite extended and quite treble, but not at MT300's level.
Preference:
-- Shells are both very nice, but in terms of fit, MT300 are one of the most comfortable IEMs: small, no sharp corners, no contest here!
Both metal shells would be really harsh for our current minus 15-17 in the last few days to wear outside
-- Overall - since MT300 are my only EST pair, and they are really easy to like in so many aspects, MT300 win over BA10.
As well, BA10 were $85-90 at their launch in 2017 (or early 2018) - unheard for KZ, while MT300 were $170, so it is also a factor.
MT300 overall.
MT300 also wins over ZAX for me and "as is" ASX.