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.
And don't you think about the fact that Lammens and Martinez are goalkeepers of different levels and the reason is that the transfer price is different precisely because of this. Manchester United, precisely because of the desire to save money somewhere, came to the fact that their level is becoming lower, first these are less famous coaches, then they look for compromises among the players and as a result they will get average results, or even below average. To become the best team you need the best players, in this case there should be no compromises.
Just because Martinez is experienced, then you called him better than young lad. Paying £40m for 33 years old goalie who have some years left to play makes no sense. Investing in the generational talent like lammen is a better decision. He has room to growth, He can stay for years in the squad, and have low salary.
Despite squad quality, you must be also caring with club's financial.
I quite understand your angle of thought, it could be the best for the club that are financially challenged, or that are planning to cut cost, but not all clubs look in that direction, because reason for their action matters, and they look beyond the money difference at times. If I were to choose myself, I will go for Martinez 100%. The age you also mentioned is just a number, is the guy not delivering at that age? Experience count here. Well, with the present status of Man U, I hope they made the right choice with Senne Lammens, the club is dire in need of a good goal keeper.