I just ordered the 507 from Vulcan Tire and originally I ordered the kit with the flow through valves but they sent me the replaceable cap style units instead. When they realized thier error, they sent me four of the flow through style. Both have replaceable batts, and both weight only a few ounces or less, not enough to throw off the tires balance.
Looking at at both side by side, I actually have started to like the flow through so even more, because it will make topping off much easier instead of having to search for the little tool to take the sensors off each time. The flow throughs allow the tires to be inflated while using the monitor to check pressure live. Be advised as others have reported that the pressure shown on the kit can be off by 1-2 lbs from an actual tire gauge.
Also, I went ahead and purchased the 507 repeater because my rig is 30 feet and at around that distance the system can loose connectivity.
Either way you go, you should be happy with the system!