What is the difference between the AmpliFi Alien Router and the Linksys Velop AX4200?

Without knowing both your home dimension where you want wireless coverage and your overall networking goals, it would be difficult to tell you which of these wireless routers would best suit your needs.

All we can assume, at this point, is that you desire a networking device that will provide coverage from one side of your home to the other. With that, the only thing I can offer at this time, is no matter which device you go with, understand the vendor's return policy in case they don't work out.

In addition to FoxFifth's excellent comments, you may also want to visit the SmallNetBuilder site to see reviews of these routers and numerous others.

In addition to the other comments, while Apple offers both of these products online and in the Apple Stores, they offer no technical support for the products. Apple will refer users who purchase one of these products directly to the manufacturer on any questions, problems, etc.

So, unless you really want to buy from Apple, there would be no real advantage to doing so, especially if the product that you want is offered elsewhere at a more attractive price.

I recently purchased the Motorola MB8611 and the TP-Link Archer AX90 (AX6600) both based on advice from Wirecutter (upgrade picks for gigabit speed internet service). My router is in the middle of a small single-story house and I don't need a mesh system (and don't have any experience with them). You may want to look at Wirecutter's Mesh-Networking reviews and look at their upgrade pick (based on your Motorola MB8611 modem support of gigabit service). See: https://www.nytimes.com/wirecutter/reviews/best-wi-fi-mesh-networking-kits/

I don't know that how their recommendation compares to the two models you mention but my experience with their router and modem recommendations has been good.