Both are the same product - polish!
SRP is generally good polish and works well if handpolished. UDS is supposed to be for darker cars but isn't used as much. Given that you should only polish two or three times a year, you can try one product for six months and then use the other for six months. Bear in mind the prep work that needs to go before it (wash, clay, dry) for best results.
Finish with a wax or sealant of your choice. AG products tend to work together well so you can apply SRP or UDS, followed by say AG EGP sealant or AG HD wax. Both available from halfords.