You made me make some numbers on this. Assuming all skills are desirable:
Nimitz: 1767 price (with 40% discount)
Camouflage Lv 1: 120 medals
Fleet Leader Lv 2: 260 medals
Ace Forces Lv 3: 380 medals
Total: 760 medals
Dönitz: 2715
Depot Ship Lv 2: 290 medals
Fleet Leader Lv 3: 500 medals
Wolfpack Lv 4: 1200 medals
Total: 1990 medals
Differences in purchase price makes Nimitz 948 medals cheaper (with 40% discount) and both have the same rank.
Differences in skills makes Dönitz 1230 medals cheaper.
So overall, Dönitz is indeed 282 medals cheaper. Besides, he has a decent mobility, while Nimitz has an almost maxed Air force attack. I see them very close, with Dönitz being better/cheaper if going for a pure navy general and then a pure air-force one.
My main problem is that I'm on a PC, so I cannot get the new navy generals. Having Leeb, Guderian and Konev (appart from Messe and Tito), would you recommend me saving for Dönitz or getting Rommel? I have now a bit over 2000 medals, so I can buy Rommel straight away or grind a bit more for Dönitz.