The outdoor VRD is a good place to start. However, the 'use case' for external antennas can be broken down into 3 questions:
1. Is it for an AP to AP deployment (ala wireless mesh or bridging)?
2. Is it for a high density deployment?
3. Is it to provide coverage that is farther away than what the integrated antennas will provide for?
There are sub-questions on each of these. But in general, that is where I would start.
For #1, if it's for an AP to AP (mesh or bridging), then the questions come down to three other questions:
A - is there a specific distance you are trying to hit?
B - Is there a specific topology (PtP, PtMP, etc) you have to design to?
C - is there some level of interference you are dealing with?
Obviously distance dictates antenna selection to a great degree. However, if the design also needs to be a PtMP, then the antennas have to be selected such that all the APs within the mesh have to be within each other's beamwidths. Additionally, if it's a dense wifi area for AP to AP, you sometimes have to select the narrowest antennas to provide the maximum amount of interference rejection. If the antennas is wide, it will pick up all interference within the antennas beamwidths. Doing a PtP with the 275 is really not a good design simple because the APs will pick up ALL the other interference around it, when the design doesn't call for it.
So the TL;DR for #1 is - Select the antennas that provide the most gain to meet your design requirements, in the narrowest beamwidth required to meet the requirement and provide the maximum interference rejection.
For #2, high density deployments are ALL about 'cellularizing' coverage. If you are trying to cover a 2000 seat auditorium, and each AP can support 100 users, you can't just put 20 APs in that auditorium (well, you can, but it will not work well). So in that case, you have to come up with a design that meets the requirements of the environment, and that WILL require the use of small beamwidth, narrow-beam antennas *as much as possible*. That provides the smallest cells of wireless coverage with the maximum amount of interference rejection between APs/cells. The BEST place to read on this condition is here - http://community.arubanetworks.com/t5/Validated-Reference-Design/Very-High-Density-802-11ac-Networks-Validated-Reference-Design/ta-p/230891
For #3, if you have either a large indoor space, or want to provide outdoor coverage for an area larger than what the omnis would cover, using something other than the omni antenna is required. A general guideline outdoors with clear open space is 100-150m radius coverage around the 275 for client coverage. However, if you need to go farther, you can look at the AP-277 (equivalent of an AP-274+ANT-3x3-D100), which can extend that coverage out to 300-400m.
To combine the above, if I need to link up two APs of coverage (use case #1) AND provide outdoor client coverage (#3), then I would use two AP-274s with the narrowest 5Ghz antenna possible (ANT-2x2-5314) and then use the omni 2.4Ghz antennas for client access on both APs (ANT-3x3-2005).
So hopefully that's a good start, but antenna selection is ultimately decided by the environment, the use case, and the requirements of coverage. The VRDs (outdoor and VHD) are great starting points.