Especially after being out of the hobby for 20 years, I would say go with the UCD. It has a WAY lighter wingloading and is easier to fly because of it. I am simply not a big fan of the GP 300S because of it's weight compared to wingloading, so I am a bit biased. Still, we had a guy join our club that hadn't flown in 30 years and bought the Patty as a first comeback plane. He confettied it on his first attemt to fly solo. It is also way under-powered with the OS 160, and way too heavy with the current line of gas engines available.
I recommend the UCD.