Difficult to imagine two twenty year old Fords being in the same condition. With that in mind, unless you need or could use the extra space of the Focus, buy whichever car is in better condition (very much taking MOT history into account).

Not sure the Focus would be noticeable more refined on main roads. How comfortable the car would be on main roads would also depend on the engine. I wouldn't be too keen on the 1.3 Duratec Fiesta with its 60bhp for example, nor a 1.4 (75bhp) Focus.

If you had the choice between average spec c. 2002 Focus and a Fiesta, both in similarly good condition and same mileage,

3 years ago we helped the wifes uncle replace his 2003 Fiesta. He had owned it from new and had covered under 34,000 miles in those 16 years. It had been serviced every year and when cleaned the bodywork actually looked OK.

But the MOT history revealed a different story. It had never failed but the number of advisories concerning corrosion increased every year and a quick look and poke under the car suggested to me that it was nearing the end of its life. Wifes uncle thought the advisories had all been repaired as part of the yearly service/MOT, he was unaware what the word "advisories" actually meant. The MOT had only 2 weeks left when it was PX'd and the dealer who took it off us sent it out into the trade. The buyer sent it for an MOT and the failure list was long and worrying. Quite amazingly it past about 3 days later still with advisories.

Just looked at its current MOT status and it expires next week. Be interesting to see what is on the MOT site in a weeks time.

But would I buy a 20 year old Fiesta (or any car for that matter), no way, rust is going to be an issue.