I'm not familiar with the cossy set up, so cant really comment on that.

The fiesta set up is probably the easiest and cheapest. Will require an inlet manifold and coil pack from a fiesta, throttle cable and a few sensors dotted round the engine bay, that all pretty much bolts on, the inlet manifold will need either an adaptor plate or an EFI head fitted. Major problem is the crank sensor, this will either require the block to be drilled out and a sensor fitted, or burtons do a crank sensor kit which bolts on to the altenrator side of the crank shaft. I think its about £120 or so. This will allow you ti run the rs turbo engine on EFI, ditching the metering unit, the distributor, the coil, losing most of the loom. Fiesta EFI has the advantage of improved reliability and slightly better fuel economy. Downside is that it costs more money to set one of these up. With an rs turbo you can do a few mods, adjust the fuelling slightly, and the jobs a gooden. Fiesta EFI requires chips and different stages to get the most out of modifications.

I am going down the other route, the more expensive route!! I've picked up Omex 600 ECU and loom for £610, getting the loom made up and the car mapped is costing me a further £350. On top of this I have had to buy the above fiesta bits, manifold, coil pack, throttle cable, map sensor etc. I have cheated a little and am using a ZVH engine. As the Zetec engine has the crank position sensor, this allows me to run efi on this with no further modifications. The advantage of omex will be getting the extra potential out of the engine. The fuelling and timing will be specific to my engine and I believe it is the best way of doing things!!

The above is my opinions and may be subject to argument.