The best security is one which stops a potential thief from even trying to steal your car, so it needs to be both highly visible and known to be very difficult to bypass.
As you say, the Diskloq appears to be the best solution currently available at sensible cost. The downside is that it's a bit awkward to put on and off, but if it saves your car then it's probably worth the minor inconvenience. (And, whatever you do, don't lock the Diskloq before you ensure that you have the correct key - and not just one that looks like it - with you. Don't ask me how I know...)