Here are my personal reasons for preferring mainstream with 1-to-1:
More and more real integration
Higher standards - the units I've seen, unless targeted at hf kids, have been pretty meh in their expectations
Units arent a good preparation for the rough and tumble of the real world
I hav seen ome really uninspiring teachers in units, but perhaps I've just been unlucky
In the mainstream class, child has more chance of accessing the curriculum - many units I saw were unite content to let the kids coast along on P levels ad infinitum
If you get a good 1-to-1, it is like having a teacher to yourself to decipher the curriculum and help your child. Apart fom atlaytimr, s/he has an adult in their corner
In a unit the ratio is not 1-to-1 so a less personalised curriculum
The unit will often say it is autism-specific, but this can be very woolly and mean either Teacch or "eclectic" - neither of which were my good for my boy (we used LSAs trained in ABA)
My hf DSD did primary ms with a 1-to-1 , went solo at ms secondary and is now as near as dammit nf