From the reviews I've seen they apparently changed the lid design to fix the rattling issue. I have not used one myself, but I've seen multiple reviews that have pointed that out. (Shrugg)

 

As someone looking to setup a new tank and looking at HOB filters I am interested in the new model. It looks like they addressed the minor issues in the prior model according to reviews I have watched. From the reviews they fixed the rattling top and redesigned the media basket. Addressed the restart after power outage issue with a new intake tube design but there was one comment in a review said it did not restart after power was off for half hour. Could have just been a water level issue for this one? The motor is still outside the aquarium which was disappointing to some but I actually think I prefer the motor outside so less stuff hanging in the aquarium. They added a prefilter sponge to the intake tube which seems like a good idea. One reviewer mentioned its a less brittle plastic body so probably more durable. The only negative change I can see is the easy access to the flow control is now under the lid. So not as accessible if you use it when feeding your fish to slow the flow. Apears to be showing up at the chain stores now and LFS coming soon.

There is a nice video you can watch on the fluval website in below link.
https://fluvalaquatics.com/us/shop/product/aquaclear

 

i have a new one, it works fine. There is no rattling of the lid. I also have an old one, a smaller model but of the old style. when i lose power, the smaller old one will not prime again when the power comes back on, i have to go down and fill the filter cavity or prime it so to speak. The new one rebounds fine after a power outage and is able to re-prime itself or at least i know its running while the old one in the same room is not after a power outage.

i have no idea whether the older one i have is an issue related to what they changed or not, but i am happy with the newer one i bought earlier this year.

 

Petsmart carries them. I wanted to purchase some, particularly the AC 50 model. I couldn't justify spending that much when Amazon currently has the "older" model for under 40 dollars. The Aquaclear 70 right now, for the newer model, is 70 dollars for just one. For that price, I can get two Aquaclear 50's. Older model.

 

It's been a few years since I used my old AC filters but I don't remember them ever having issue priming after a power outage. Almost the opposite it was very good at restarting. The water level would be a contributing factor though. I tend to keep my tanks topped off.

Sounds like it might be a good time to scoop up some of the old ones at a discount to keep on a shelf