If you're not bothered about getting a complete donor car, then either way will do you just fine, there's plenty of both Ford and MX5 diffs, gearboxes, engines etc available and aren't prohibitively expensive. There were a million or so Sierras made, so there's truth in what Robo says.
If you want a full donor (e.g. for age related plate) then you will find more suitable MX5s than you will Sierra (don't forget the youngest Sierra you'll find is still nearly 20 years old). You can negate (or significantly reduce) the purchase cost by selling off unwanted parts but the MX5 market in particular is full of these and it's more difficult now to achieve the prices of a year or two ago. There are still some parts that command strong money (e.g. decent condition front wings, JDM tuning parts) though.
The MX5 bits are a little lighter than the Sierra ones, you will need 15" wheels to clear the Sierra based Roadster rear uprights, the front Sierra uprights will need modification and mushrooms to fit.
You do have a much greater engine choice with Ford running gear though (depends on what you want from the Roadster). The MX5 running gear can handle approx 250bhp from what I've read (if a Torsen diff is fitted, which is apparently rated to 300bhp). A Sierra diff, I've been told, can take a similar amount of power/torque.
There's plenty of options and knowledge on upgrades for both Ford and Mazda, although there is a greater choice of options for Ford IMO.
I went the MX5 route. The reasons why are...
1 - I wanted a complete donor car
2 - an ideal donor car (scruffy but decent condition running gear) was for sale at a fair price less than 10 miles from home
3 - market prices for the unwanted parts were very good
4 - the MX5 gearbox is widely recognised as having one of the best actions ever
5 - OEM Mazda wheels are generally very light and are very cheap
6 - the MX5 is a sports car, so the components were designed and built with that in mind. The Sierra was designed as a repmobile/family car, so may have had a slightly different design brief to Mazda's. Some people have the opinion that it's more satisfying taking a repmobile and turning into a sports car, rather than starting with sports car stuff in the first place. Fair enough.
WRT Robo's comment about possible issues with the MX5 diff. I don't think it's an issue per se, the front of the diff needs a custom mounting to replicate the PPF frame which links the diff and the gearbox together.
HTH
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