We're speccing up some 10GbE switches for integrating a few older servers into our Equallogic SAN, and we're noticing quite a price gap between SFP+ and Copper (Cat 6A) equipment (Dell 8024F vs 8024).
I'm not really sure what the real-world difference is between the form factors. The Dell guys tell me that SFP+ has a lower latency, but couldn't tell me much more than that besides that our M1000e and PS6010XV chassis's only comes with SFP+ uplinks (and SFP+ is substantially cheaper).
Best Answer
The latency is basically negligible. The 10GBASE-T has a latency of <1microsecond. SFP+ has less latency in itself, but SPF doesn't include some of the physical transceiver (which may or may not add latency); hence the need for a physical module (or direct copper cables).
The biggest differences are price, as you've noted, and distance. SFP+ Direct Copper the cables have to be <15m (10m for certain cables). 10GBASE-T goes the standard 100m. Cat6 cabling is quite cheap (compared to other 10G cabling), and I would suspect that the equipment manufacturers "make up for that" in the price in addition to 10GBASE-T not being as popular yet.
The 10GBASE-T standard also uses more electricity (which causes both the increased distance and latency). The extra amount used isn't normally a factor.