[meteoestarreja]

Well, at 5 meters you almost don't need an antenna (just kidding)
If you are going to choose an ham antenna, you could try to find a mono band antenna just for uhf, you will get a shorter antenna (since vhf being a lower frequency, needs a bigger antenna - like I said in last post). It's only about the size of it, being a dual band there is no problem.
What you said about working 3Mhz above, that's not a problem, the referenced frequency it's just a reference of the band, in amateur bands, you don't "work" on a single frequency, when 430Mhz it's referenced, it means that the antenna will work at least from 430 to 440 Mhz, having the best results somewhere around the middle and getting increasingly worse as you getting out from that "point".
That antenna looks like a clone of Diamond and I guess that it won't have any gain, but yes, it should perform ok.
You should also pay attention to antenna's connector, avoid different connector because as more adapters you use, the higher is gonna be the loss, and like you said, with 25 mw, you don't want to lose that much.

Also in the pcb, Werk_AG didn't have a long path between TX module and antenna's connector and that's good, the pcb "route" being an ordinary electrical conductor and not being shielded, the pcb itself in that small path became an antenna, the shortest, the better