The parcels are located in Franklin Park and border Olde Town East, so it's a VERY hot location. 

Delaware is a small enough state that I actually started calling places in surrounding Maryland/Pennsylvania/New Jersey because everything is so close on the state borders.

Primarily Oklahoma, but have purchased a few in bordering states. 

That fact, combined with the amount of time it covers, causes it to have limited usefulness that borders on irrelevancy.  

Must it directly border the "city limit"? 

Illinois will get a lot of the benefits, with it being placed so close to the border, but Illinois didn't have to shell out the billions in tax breaks and subsidies that Wisconsin did.  

Some friends of mine who rent to members of the military enjoy the fact that they basically are guaranteed the rent for the terms of the lease (generally two years). 22307, 22308, and the 22309 zips are at or border Ft.

In your particular case, it will boil down to a few things: who you invest with in the US (most important), solid cross-border CPA and investment strategy. 

Perhaps I should focus my efforts south of the border. 

George Dube and Peter Cuttini both CPA that work together across both borders.