I've owned a 306 Rallye, a 172 Cup, and a GTi-6 as well, trust me the Clio is not massively faster in the real world...
The Clio is quicker off the line due to being lighter and the closer gearing, but the 306 is stronger at higher speeds due to the ratios of the 6 speed box. Handling wise, the 306 has the better chassis, it's more composed at speed and confident over ruts/dips/potholes, cross country I could push harder in the 306, wheras the Clio is better at tight twisty lower speed stuff, like a go kart.
My 1/4 mile times:
Rallye - 14.8secs @ 93mph
Cup - 14.5secs @ 93mph
GTi-6 - 15.0secs @ 94mph
(all 3 standard apart from a filter)
Not a massive difference there, look at what 1/4 mile times most 172/182's get and the 306's are right up there!
Remember that the 306 is an older generation of car though, they came out back in 1996 now and back then they were king, just a shame Peugeot went off the rails.