from a business only view...i guess the mags as things stand are by far the greater attraction simply due to crowds and league status, the down side to that is that it will be a lot harder to turn the investment into a substantial profit, something that realistically could only happen with top 6 finishes and european football almost guaranteed every season and everyone knows just how hard it is to reach that goal now.
as an investment, sunderland is by far the best bet, mainly due to a very possible doubling of the crowd, far less will need to be spent to improve the clubs standing, once in the premier for a few seasons then the asking price will rocket (the next step will obviously be similar to the mags or any other team bar the top 6) but the difference could be that the land fill can not be increased in capacity, iirc they went as far as they possibly could to make sure they had a bigger capacity than the SoL whereas the SoL has the potential to increase to a 60k capacity.
so, as already said, it really comes down to what a buyer will want...it seems many of the rich just want a football club to play with and want the prestige of it being a premier league team, losses do not overly concern them as they have more than enough money to replace any losses through interest they earn on other money...but these days there are also a lot of consortia who like to find a potential short term/huge profit investment.