Hi,
I have an EdgeRouter Lite 3 and would thoroughly recommend it if it meets your needs. It essentially implements a fully fledged iptables based firewall with QoS and hardware acceleration capable of gigabit throughput in a £100 box. It's definitely not a product for a beginner, and takes a fair bit of knowledge and time to configure, but once configured it makes an extremely powerful firewall with decent throughput (note you will not get gigabit throughput for VPN usage). Mine has been extremely reliable and stable, and never requires a reboot. To say I have been impressed would be an understatement. I specifically bought mine for it's high throughput when switching to a fibre connection - I had been using older Netgear firewalls which were no longer capable of handling increased fibre speeds.
The 3 ports on the EdgeRouter Lite 3 can be configured any way you like, but the most common configuration would probably be to connect one port to the WAN (modem), one port to the LAN (maybe to a switch to further distribute) and the 3rd port could be connected to the wireless AP (so 1 x WAN, 2 x LAN). This type on configuration allows you to fully separate the LAN (wired) and wireless portions of your network, if that is important to you. Particularly useful if you want to grant guest wireless access to your network.
I personally use the 2 LAN ports to fully separate my son's Windows PC from the rest of the network (Linux) so that when he inevitably gets infected he is not a risk to the rest of the network. I can also use the QoS features to cap his bandwidth if his usage is impacting upon the rest of the network and the firewall to block traffic during hours he should be sleeping
There are quite a few technical differences between the two models. The ER-X model contains an internal hardware switch for all 5 ports whereas the Lite does not, and the ER-X supports PoE if that is a consideration for you. Other than that, the main difference is in performance - the ER-3lite has true gigabit throughput maxing out at around 1300Mbps, whereas the ER-X will max out around 600Mbps according to user reports (it will only achieve gigabit throughput at very small packet sizes).
Please do also check out the Ubiquiti support forums as they really are excellent with some very knowledgable users there.