Is the UFIP Effects Swish China 20" worth buying, or should I consider an alternative?

The UFIP is the less expensive of those two China options, but price and diameter alone won't tell you which suits your kit. The supplied details don't establish its sound, sustain or playing feel, so don't buy on the Swish name alone.

The Meinl Byzance 18" Extra Dry China is the most relevant alternative here. Compare recordings made under similar conditions—or, better still, try both with your own sticks. Listen for how the attack and decay fit your playing, and whether the cymbal overwhelms your existing setup.

If you're open to a different kind of accent cymbal, browse the used crash & ride cymbals list as well. A crash isn't a direct substitute for a China, so choose the sound you need first, then weigh up the deal.