I own them both and for me it depends on the reason flying. Do you want a jet that is great for aerobatics and super responsive then I’d go MB339, if like the idea of carrier operations, a more modern HUD then the T-45C Goshawk.
Both are really good and I’d highly recommend watching reviews on them both to decide.
Edit: just realised the date on this… no idea why showed in my latest 
but I guess may help someone else