Regardless of what mode you choose, repeater is the worst as it will impact performance, most other modes are custom to dlink or not standard. If you want more speed from a point-to-point link you need better antennas, perhaps moving over to 5Ghz is even better.
As it is only a link and transparent to the users of the network, there is no need to stay on 2.4Ghz anyway external antennas will not impact the units, regardless of hardware, brand etc. They will just make if go faster, provide a more stable connection and so on. Lots of hardware to choose from, antenna wise.
One important aspect if you are on standard WLAN equipment, dlink, netgear and so on, if you can put the antennas directly onto the units, or use a antenna cable that is as short as possible, this will help to avoid signal drop due to signal loss in the antenna cable. Otherwise you will loose a lot of signal strength.