I admit Martinez is a quality goali. The problem is that Villa's asking price is too high. How the hell they asked £40m for him. Also paying £200k weekly for him is just waste of money. Buying Lammens is one of their good alternative. They have spent a lot, and should not be overspending on their new goalie.
The good thing is Martinez when with Argentina but when with Villa, Martinez also often does blunders that harm the team. Compared to Onana, yes Martinez is better as a goalkeeper.
Besides what I know, Manchester United only made Martinez an option if Lammens was not successfully obtained, so it was very clear that Martinez was not their top priority.
When Manchester United got Lammens, the reason for the salary and costs requested by Aston Villa was the reason behind the failure of Martinez to Old Trafford.
£40m is a very high price for Martinez, and I don't think it's worth it considering Emiliano Martínez is already old (if Man United buys him, it is likely that he will retire there in the future, or if he leaves one day, his price will drop drastically). Therefore, Man United chose Senne Lammens because the price was much cheaper, and it could also be an investment.
I'm just a bit surprised why Man United didn't try to go after Donnarumma, though there were rumors that Donnarumma was interested in playing for Man United (before he was linked with Man City). Besides that, Donnarumma's price is not that high, and Man City only spent £26m to get Donnarumma's signature.