In "N" mode, you can get upto 300Mbps with a pair of those Chinese routers, there are too many variables to calculate, but with those antennas you have at 300m you should get at least 150Mbps with the Chinese routers.
My feelings are if you were using a low power AP before, you already have the antennas in place, the D-Links will be fine, but they are end of life here in the states, so little or no firmware updates. BUT the fact you can put the 3rd party software on them is a plus, really don't care about software updates in that case. If you did not have the big antennas already, I would say go with more power. But since you already have the antennas, I think you can't beat the D-Link quality!
The Chinese routers vs the D-Link, the D-Link is a high quality product and supports the 3rd party firmware, the Chinese never will have a 3rd party software. Heck, I have written for tech support on the Chinese ones, there is none!
If you really need the power later, you can always go with a cheap amp for the dir-600 and have the best of both worlds, quality and power!
The reason I did not go with just amplifiers and a good D-Link router is because I have the DIR-825 dual band, 2.4/5ghz. No dual band amps yet, for you, the amps and having quality hardware to back them up would be my choice.
(I would go with the D-Link)
The other reason I bought the DIR-825, IPV6 GOLD certified, and I use IPV6 everyday.
Another downside of the Chinese AP's, I would NOT trust their firewall. Good as an AP, no way for a firewall/router on the internet. I would worry about back doors if they were used as a firewall.
You have a pretty amazing system, I don't think you will go wrong with D-Link..... You can't beat a good antenna system!
V/r,
Pat