Not seen either of these in the flesh, but here's my view (other views are available!)
The cheap one is a 30A relay. It uses 30A cable, and 20A Fuses. This is sound enough, however a flat L/B will draw way more than 30A on charge. Your alternator will happily supply 70A or more if the battery requires it. If this is the case, the 20A fuse will blow, and the L/B won't get charged.
This cheap one is also just a charging solution - it doesn't include any DC distribution from the L/B or DC injection into the van's original circuits.
The dear one doesn't state the relay, cable or charge circuit fuse ratings. The cables look on the thin side to me, and the fuse holders look like the regular type with yellow fuses, so are 20A. Same flat battery charging issue therefore applies as with the cheap kit. However, this one does include a DC distribution panel, and a loom designed to plug into the original fuse panel, which is a very neat and easy way to swap circuits of your choosing over from main battery to leisure battery.
The original Willinton kit used a 100A relay, 70A cable and 50A fuses, so there is no chance of overloading the relay or cable, but gives you the ability to charge a very flat battery (I have done with mine, and nothing blew). I have worked on a Bongo with another (£17 style) system, which kept on blowing the charge line fuses until you charged the L/B in the house on the mains and refitted it.
As with everything - you pays yer money etc. (The Willinton kits were around £100)
Now, if you have a solar panel as well, you 'should' never have a flat L/B, so will never need a heavy-current charge system.
If you want to do leisure batteries on the cheap, you can buy a marine isolator, and wire it up with suitably heavy cable. This allows you to start on the starter battery, run on both (to charge them) and then go onto the L/B only when parked (and isolate both if required, like over winter) Like
this. Of course, you have to mount it somewhere, and it's a faff.
Even simpler - you can just link the 2 batteries together with a suitable heavy duty cable, and fit a
single line isolator in the main battery earth line. Simply isolate this when you park, and everything will work off the L/B with no chance of flattening the S/B.
For what it's worth, I have a 113Ah L/B, Willinton kit, 80W solar panel & charge controller, and a mains PSU / charger for hook-up.
