so ive been running a diamond X300 at my place for about three years now and honestly its been fine for what it is, decent gain, no complaints. but im moving my main station to a hilltop property i have access to about 12 miles outside of town and i want to do this properly this time. the location is exposed, probably 900ft elevation gain over the surrounding area, so wind loading is going to be a real concern especially in winter.

been looking at the Comet GP-9 and the Diamond X510 mostly because i know both brands and trust them but the price gap is noticeable depending on where you buy. the GP-9 is rated something like 12.5dBd on 70cm and the X510 is close but not identical on paper. has anyone actually A/B'd these in a real installation or am i just splitting hairs at this point. my main use is going to be linked repeater access and some simplex work, not contesting or anything serious.

also curious about the mounting situation — i have a 2 inch schedule 40 pipe already at the site, about 10 feet above a concrete pad. wind here gets nasty in december through february, we've had 80mph gusts on record up there. not sure if either of these antennas is actually rated for that or if the spec sheets are just optimistic.

ran a GP-9 for about 4 years on a similar exposed hilltop site, roughly 1100ft, and it held up fine through two winters that knocked other stuff around. the wind loading specs on the comet documentation are actually fairly conservative in my experience, the antenna itself isnt the problem usually its whatever youre mounting it to. your 2 inch pipe setup should be solid as long as the base mount is done right and you dont have a lot of slop in the mast clamps.

as far as the actual RF performance difference between those two antennas in your use case — honestly for repeater access and local simplex you probably wont notice a meaningful difference. the extra dB on paper from the GP-9 on 70cm is real but its fractional and your location advantage is going to do way more work for you than a half dB of antenna gain. if price is a factor just get whichever one is cheaper at the time, both are solid antennas.

i had a X510 up for two winters in northern maine and the radome cracked on the second one, might have been a bad unit or maybe the cold just got to it, idk. comet felt more solid to me physically when i was handling them side by side at a hamfest. that said i know guys running X510s in worse conditions with zero issues so maybe i just got unlucky. for a permanent hilltop install id probably lean comet just based on that one experience but its not exactly scientific.