As already noted, the Record and Chorus models have ball bearings on the main pivot shaft, while the lower levels have a bushing. The distinction with Record is the antifriction coating on the rachet. Personally, I think the Record lever shifts noticeably easier and the cost difference (sometimes only $20) is not that great, so I get the Record levers, regardless of what else goes on the bike. If your budget is tight, Centaur will work just fine. All the other internal parts are the same as Chorus. The '07 Centaur and lower models are a different product, no longer having the ability to make multiple shifts to smaller cogs with the thumb button.
Do some on-line shopping to get the best price. I got my last pair from www.11speed.com
for about $50 less than most US sources. The problem with 11 speed is they've doubled their shipping costs in the last few months, so you need a big order to save enough to be worth paying $36 in freight for a modest weight package. I bought 16 Michelin Pro Race2 tires from them for $22 each. That made the order worth it.