SRX5308 has 512MB of RAM ER6120 only has 128MB DDR2 of RAM.
Over inflated 200,000 concurrent connections on SRX5308 but goes to show ou like with ASUS reported 300,000.
My ER6120 ran for 6 months without any issues. I have mine on APS so it's never off until last month lost power from 1am to 8am battery backup good for 1 hr. So that's the first time because of bad underground power cable.
Other than that the ER6120 runs without any problems. I don't use VPN here so if you require VPN you might want to get SRX5308. ER6120 rated to Enterprise Network not any thing you would pick off the shelf at your local Walmart store not there even if you look for it.
I still haven't done any upgrades only once when I got it.. I always recording so the 4x Network HD 1080p tuners are going. I am not shutting down anything for now.. Heavy Online Gaming, Heavy Media Streaming, File Sharing, Network Printing. File transfers of 1.6GB goes quick here.
Network task are just fine.
Downloads with the 132mbps and 21mbps for uploads. No issues.
Your needs may vary than ours here.