Did you check if the Infinity speakers were 2 or 4 ohm before cancelling? At a good price, 4 ohm Infinitys would sound ok and be a nice in-between step to a nice audio setup later on.
I like the G-series 6x8's for powering off a head unit since they are just 2-ways (like the more expensive D-series) with low enough power handling (30W RMS) that even dinky 18W x 4 head units can push them hard enough to vibrate the rear view mirrors with bass. The cloth surrounds also make it easier to get the most out of them with little power.
Funny thing is, you can get the A-series 3-ways (main differences being the 3-way design, rubber surround, and 40W RMS power handling) for about the same price. Wal-marts sell them for $59.99, and you can find them online for about the same price or just a few bucks more than the G-series.
Car audio shops sell them too, so if you want to stop by a Pioneer dealer and listen to a pair of each with your type of music to see if you can tell any difference in the sound, you could do that. You might even find a dealer with some old stock 6x8's for really cheap.
Otherwise it may come down to price, shop around online and see what you can get a deal on. Either of the Pioneers will do great. I think the differences are pretty minimal, the 2-way design and cloth surround of the G-series make it best for getting the most from a little amp, the 3-way design of the A-series might make it a little smoother on highs and the rubber surround makes them a little more durable in the doors.
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