Hadlee takes more WPM than Mcgrath, and does so faster considering his era. I don't see a reason to believe Mcgrath would take more WPM in the same team. Call it even for the sake of argument, although this is doing a bit of a disservice to Hadlees vastly superior record in this regard.
Considering his era, Mcgrath takes wickets cheaper. I'd say the difference between them would be that Mcgrath would go at a marginally better economy rate (mostly due to the extra bounce), which would mean marginally cheaper wickets.
The difference in average would be small. Half of Mcgraths career was the 90s, similar enough to Hadlees time. Mcgraths 2000 performances were impressive, but even if you think 2000s Mcgrath would average around 18 in Hadlees era, or that Hadlee would average late 20s in the 2000s (both of which are implausible IMO), this is a 2 rpw difference between the two across career.
Assuming 4wpm in an ATG team, this is an 8 run advantage per game. Probably a lot less, because this calculation is extremely generous to Mcgrath in every way.
In contrast, Hadlees batting would conservatively add an extra 30 runs to the team, factoring in his own runs and partnerships (think of the value add from avoiding a tail collapse when there is a rampaging Sobers for example). This is a huge swing.
Put another way, the average margin of victory between two top teams might be 150. Hadlees extra batting would win one in every 5 matches on average. It's hard to imagine how the advantage provided by Mcgraths bowling would be comparable.
I would hesitate to make this type of comparison with confidence between any other two players. But these two are so similar in style and quality, and it's hard to see them performing very differently with the ball. One of them is one of the best bats going, and one is close to the worst. I can't see a reason to pick Mcgrath.
