The strength of support is likely more important than a dB or two of difference in radio performance. The better phones on the market generally have decent radios and RF design and the differences will be small.
The huge differences are in how complete the vendor's kernel source and driver/proprietary-bit drops are today, and what they might look like for LemonDrop or whatever it might be.
"Open Source" doesn't mean that you can necessarily compile a working version from scratch, even of just what was shipped, not to mention some custom variant.
A phone like the Nexus 5 is much more likely to have good source and future support than, on the opposite extreme, something like the various Chinese phones that don't even have the legally required GPL source and build toolchain available.
Read the various phone forums related to the phones you are considering. Also read the dev threads between the pretty pictures -- are the dev's "politely swearing" at the vendor because this or that major feature just doesn't work and they "can't get it to compile" or lots of "took the code from [some other phone] to get it to work" statements? I've personally done enough swearing at Some phones' crappy developer support to leave the galaxy entirely.