Well, I had the old one, and bought the new one.
I can't tell the difference in any meaningful way. I use it for Vessel / YouTube / Twitch streaming.
I might notice a difference were I to stream 1080p Bluray(ish) quality content from my NAS. But I use DLNA (serviio) for that because WiFi and lack of DTS audio support makes the Chromecast a much worse option than streaming direct to my TV via Ethernet.