IMO the RAC system is a better system. It has GPS which neither the retrieve nor the monitor have. To get this in a tracker product you would have to go for the more expensive Horizon. The GPS means that if your car is stolen, they can track it to its location instantly. The Retrieve and the Monitor both rely on a radio system. This means that there has to be a police locator vehicle within a reasonable distance of your vehicle so that it can pick up the signal and track it. The other main difference is that the RAC trackstar can be removed and refitted as many times as you like. This makes it a much more financially viable solution if you change your car quite regularly. Cost wise the RAC trackstar can be had for a similar price to the Tracker monitor and the subscription rates are the same except that you have a choice of terms. Monthly, Quarterly, Annually.
Hope this helps.
Gav.