Uh, sorry, but ugh! A sub based system is a much preferable option. Pay it once a year, get it over and done with.
You never want to make payment an option. CoH is a product people have been willing to pay for, for about 8 years. I see no reason to change that if the game gets picked up by someone else.
By all means, have extra stuff available, but you want to cover your expenses with subs. Any extras from the shop are extra.
But that's just me, I'm quite sure I'm in the minority. I can say, if whomever picked up CoH used ads in that manner, I'd skip it.