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.