They run different hardware.
One thing you have to propagate to your customers is that DrayTek routers don't use mainstream chipsets, so its not one fast CPU to do all, its different parts that accelerate certain tasks depending on the target use of the device.
Hence the big differences in performance between the different models.

An example to give to the customer is this:
WAN Throughput:
Vigor2130: 800mbps
Vigor2912: <100mbps (that is, less than 100mbps, 1/8 of Vigor2130)

Do the numbers above matter to a Unifi 20mbps user? No it won't make a difference both models will perform the same.
Do they matter to a Unifi BIZ100 100mpbs user? Yes they do, Vigor2130 is more suitable for a 100mbps line.

But, again:
Vigor2130 VPN throughput: about 2mbps
Vigor2912 VPN throughput: 70mbps

etc.

The above happens because Vigor2130 has a chip for NAT, which makes it fast in WAN throughput. But VPN is done by the main CPU, so its only 2mbps.

Vigor2912 on the other hand has a chip for VPN, so it can manage 70mbps VPN throughput, but in WAN throughput, its significantly lower than what Vigor2130 can manage.

From here it should be very clear to the customer how DrayTek routers differ. Explain to them that DrayTek never puts software limits on the hardware.
The performance they get is what the hardware can manage.

A major difference among models is also software, Vigor2130 is focused on media features, like DLNA server, itunes etc.. all designed for pro-sumer or home enthusiast.
Vigor2912 is more focused on business features, like more thorough VPN implementation, object model firewall, Certificates etc. Those are irrelevant to the home user.

If the customer is a home user with terabytes of downloads every month and streaming HD data from NAS etc and wants full "gigabit" experience, Vigor2130 is the one to get.
If interested in Dual-WAN capability or need fast VPN/other business features then Vigor2912.
No device is better than the other, both excel at certain things. The question you should ask the customer is "what do you need?"
Understanding what the customer needs then suggesting the right router will ensure his/her satisfaction with the end result.

Good luck.