I may upgrade to this since I need a duo. For 17.5 stock racing, are there any practical differences between the two? Thx in advance! May sell my reaktor 30amp charger (rebranded Icharger 3010b) which has been an awesome charger for one of these but don't know if I can justify the prices...

Take the 406, unless you need more than 6S.

406 is also a heck of a lot smaller too. Saves space in the pits.

I ended up getting a 4010, as it was $290 new, and figured it would have better resale value. While I'm here, anything notable that one needs to know about 1st steps. IE: don't plug in certain way it will blow up, etc.?

Make sure your power supply is completely wrapped and separate from the charger. If your charge cables touch the P/S while plugged into the charge, it'll smoke a channel.

Make sure you ALWAYS buy from progressive RC or their dealer network, this way you can use your warranty.

I taped up the temp sensor port so that I don't accidentally plug the balance cable into it (assuming if you are using the smaller 2s balance port plug)