My take:

A transponder lets those with TCAS (usually airliners and biz jets) get immediate in-cockpit alerts to divert. ATC can also alert any aircraft on IFR or Flight Watch verbally of conflicts. Your transponder will do NOTHING to let you know who is about to skewer you (unless you happen to be in comm with ATC). Best if you fly near Class B/C.

A PCAS lets YOU know who is near you with a transponder (includes GA, Biz jets, and some gliders, if equipped). A pure PCAS is relatively cheap on eBay. PowerFlarm as a built in PCAS. Great for flying near busy GA airports.

PowerFlarm has a two kinds transponder detection. One is a simple PCAS function. The other is ADS-B (presently only some transponder equipped aircraft have ADS-B capability) which gives you additional location and identification information. Great for flying near busy GA airports.

PowerFlarm additionally has Flarm-to-Flarm alerts which gives each flarm equipped aircraft distance, direction and ID. There are very few Flarm equipped aircraft in the US but numbers are growing. The flarm-to-flarm protocol is also clever in that it filters out non-threatening encounters (i.e., in a gaggle or flying with a buddy on the ridge). Best for glider contests and good for gliding areas if you have other gliders in your area with Flarms. Useless if they don't a Flarm too.

Having both a PowerFlarm and a transponder covers you the best for all airspace. I flew for a while with only a PCAS and was impressed enough that I wouldn't fly without at least a PCAS. I recently replaced it with a PowerFlarm. I got PCAS hits but only one Flarm alert so far. As soon as I save enough pennies I'll be investing in a transponder as well. Probably the Trig21 as it has lowest power consumption and is compact with it's built-in altitude encoder. Having the tiny display separate from the core electronics box also makes cramming it into tight panels more likely.

-Jim