I do not have the extend, so I cannot speak to how well it works, but I would have got it, if it had 4 tuners.
I do like the 4 tuners of the quatro, I don’t usually max out all 4 tuners simultaneously however it does occasionally happen when multiple family are watching and/or recording different shows at the same time.
My 528x can manage the on the fly transcoding, but I really wish the quatro had the built in transcoder. Your 626 should have no problems with the on the fly transcoding either.
One of the things on my todo list is to handbrake all the existing files down to lower quality/size x264 encodes.
So in my opinion, for you it comes down whether or not you expect to need more than 2 tuners.
Or I guess you could buy additional extends if you need more later.