I've worked with a bunch of different types of firewalls and routers and although Sonicwall is not one of my most favorite products for some applications it does do something right at a fairly reasonable cost. The load balance option on the TZ170 works pretty good and it's easy to configure.
The unit can be configured to load balance in active/active mode, active/passive and active/active (this is my favorite). In active active, you can set the amount of data that passes though each ISP connection using percentage based or even load based. So with these configuration options one can split off data by preference or by bandwidth availability for each connection.