IME, at least this year, when booking AC flights on UA.com, as UA codeshares, Ive has seats selected for me for free, even on lowball fares that are almost 100% certainly equivalent to standard fares if booked through AC, and I beleive in both cases so far that was the same pricing as with AC. Now, for example, got row 12 on a CRJ when seats further forward that we’re available. Also IME this year, booking with Chase points on 016 stock on UA codeshare/AC flight didn’t get me a seat selection at all, and I had to call AC for it. When calling to get or change seats I never had an issue, though don’t know what would happen if traveling on my own as opposed to with a kid - on standard fares, at least, AC will give seat selection to those traveling with a child on fares that normally require a fee for it (and IMO, good on AC for this).