I dont think its a Vs as much as what turns up. You may read the sierra is in decline but that is not the case , there are still plenty about with spares etc and that will go on for years. The Sierra running gear can take more abuse if you are planning upgrades, the mx5 running gear makes for a very complete package but might have a question mark over the rear diff arrangement. Only time will tell.
bob ( getting ready to be shot down in flames)
