In the sort of price range you are looking at, the price performance kings are:
Athlon X4 860K
As standard the Athlon X4 860K is a stronger performer. It is a 3.7 Ghz quad core CPU based on AMD's Kaveri core, which offers higher performance per clock cycle than previous generations (still some way off what Intel are achieving however). At £53 at the time of writing, it's a steal IMHO. The G3258 is very popular because of it's low price and high overclocking potential, which tests have shown can bring it up to the level of an i5 in some scenario's. It'll always be a dual core though.
The main difference between the Pentium and Core i3 series is the i3 has Hyper Threading (two physical, four logical cores) which helps with multi tasking and running multi threaded software. It also has more cache memory. One disadvantage of just looking at a single benchmark number is it doesn't tell the whole story. Hyper Threading really helps in some situations - http://www.anandtech.com/bench/product/1192?vs=1256 (not the exact models you mention, but to give an idea). However I think the 860K at current pricing is better value.
The AMD chip would need an FM2+ board, for example this. EDIT Should also add that unlike the Pentium, it has no integrated graphics solution though.
Edited by jonuk76, 17 December 2014 - 05:58 AM.