If you're happy with the 050As, I'd stick with them. Two rears will be cheaper than a whole new set of budget tyres.
452s are being phased out now anyway (thank god), but have noticeably less grip and feel in all conditions than the 050As. Still better than super cheapy tyres, though.
Vredestien Ultrac Sessanta are miles better than the 452s, and are only a fraction more expensive. They're a no-brainer at that end of the market if you do decide to replace all four tyres.