I'd go with the company that has the best reputation for developing people - as that will be you. Both M&S and Mars have produced several FTSE CEO/CIO leaders so seem good prospects.
If you like free chocolate - Mars
If you like a discount on socks/pants - M&S
If you like travel - BA
If you smoke - try British American Toboacco
If you drink (a lot) - Diageo
If you want to see your co. logo everywhere - SAP