Site to site vpn solutions

The Sonicwall is a rock solid appliance. I believe your random disconnect issues will probably be due to the type of circuit/service you were receiving more than the hardware. I can tell you, having a VPN connection across a T1 will offer more uptime and less problem than across DSL or cable, that's just plain to see.
Granted, hardware can have it's quirks, but barring a mechanical malfunction, these level of devices tend to just run and you often forget about them...getting on a stable software/firmware code is important also.

Any manufacturer playing on that level, ie... Sonicwall, Watchguard, Cisco (not Linksys product), Juniper, etc... will offer you your best commercial grade product built for uptime, throughput, and security. Players like Cisco/Linksys, Netgear, DLink, Belkin, etc... while having excellent products, their roots are from the home/SOHO beginnings and are usually geared toward that end. I say usually as each has built some small business level product line that works well and might suit your needs. You generally get what you pay for would be my best offering. My two cents of course ;-)