Thanks for the input, my intention wasn't for this to become a comparison of the two models, but even so, we're already using the 3400 and it has been great; love the form factor and performance. I don't see why anyone would buy the 4200-PX when the 3400 has just about twice the backplane and overall throughput and comes with four 10-gig uplinks and QSFP and typically costs less. I think the only thing we've missed so far is the LCD screen which can sometimes come in handy and also the 'T' models have 8 PoE ports which is nice when you just have a few PoE devices.
I didn't realize there was a formal terminalogy for the configuration differences (ELS), but definitely found that out the hard way. I'll check out the conversion tool for this next upgrade as the configuration is a bit more complex than our previous replacements.
Per your comments about the warranty on optics, we should be covered so we'll just stick with the ones we have. However I assume the RMA process would be similiar to other removable components such as power supplies and one time when we had a power supply fail, it was purchased on a separate order from the switch itself, so they gave me a hard time when trying to RMA it and they ended up replacing the entire switch instead of just the failed PS because it was easier than trying to find the necessary documentation to prove it was associated with the switch. All this to say I wouldn't be surprised if I have to jump through a few hoops to get them replaced in the unlikely event they fail, given that they've moved from switch to switch.