Of the three you've mentioned, FreeSRP would NOT be the one I choose (not for the price asked), not having a community and, judging from one of the demo videos, has systematic noise in one band, I don't know where it would have come from without having one in hand and testing, but it could have been RF routing and/or isolation related or possibly a poor choice of some component used in the design.
The bladeRF has a fair following and would be the easiest to find someone else who might be trying to do (or has done) the same or similar things you are and/or could help you solve any issues you might run across. The bladeRF also has a few RF and DC test points, which although nowhere near as comprehensive as the test points on the hackRF, could still prove useful.
From what I can tell from my brief look at photos and video, the Ettus looks to have the best RF layout (in my opinion) and assuming the videos showing the incoming RF spectrum of the various SDR's (FreeSRP, bladeRF 2.0 micro xA9, Ettus USRP B200) I've talked about thus far are truthful (meaning no filtering, signal conditioning, or post-processing), then the Ettus has the cleanest noise floor.
A PXI, as Andrey_irk said, would be another great solution, although would be the most expensive option.
In the end the decision is yours, and in my eyes, from what has been said thus far, comes down to the bladeRF and the Ettus.
Good luck, I hope whichever option you choose works out well for you!
Best regards, 73
Trent Spencer - KM4YTU
EDIT: Forgot to include what I meant by "truthful" when regarding the spectrum of the radios, added to post.