Warm climate, family car, do you go a Group III 5W30 synthetic for a bit more coin, or a Group II 15W40 A3/B3 mineral for a bit less coin ?
For the Group III 5W30 synthetic, would you select the A3/B4 or the Dexos1-Gen2 version, given the same price ?
Okay, I'll have a go at answering your question but first I want to talk about guitars...
Eddie Van Halen, the virtuoso rock guitarist, famous for his 'brown sound', died recently. I heard Brian May (Queen's guitarist) talking about meeting EVH for the first time. He recounted how they did a guitar swap. Brian played EVH's 'Frankenstrat' (a customised special, made from bits) & Eddie played Brian's original, homemade 'Big Red'. The result? EVH sounded just like EVH & Brian May sounded just like Brian May. The intended moral of this story is that what you might perceive as radically different might, in practice, be nothing of the sort.
Okay, back to oils. Can I assume it's a gasoline car?
Group III 5W30 (presumably A3/B4) or Group II 15W40 A3/B3? For me, the 15W40 is the better lubricant. Reasons are lower Noack, less VII, marginally more ZDDP & possibly less ashless dispersant (not that big a deal for gas engines, more significant for diesels). Also Group IIs are 'greener' to produce than Group IIIs. Also I'm notoriously cheap!
Group III 5W30 full synthetic A3/B4 vs dexos1-Gen 2? For me, the A3/B4 oil wins BUT these two are so close, there's not a lot to differentiate them. I'd opt for the A3/B4 oil because it HASN'T been put through the US fuel economy tests. I don't like these tests because as often as not, they 'encourage' formulators to drop the oil's CCS viscosity to silly levels to cheat the test. Lower CCS means more VII & higher Noack. The A3/B4 oil might also have a higher ZDDP content (but not guaranteed) & a higher TBN.
That said, both of the rejected oils would give better fuel economy & we really do need to think of the children...