I'm going to make an unpopular recommendation: the Celestion G12M-70 Modern Lead. Most guitar players hate these speakers, but since you need a speaker for both Fender and Marshall sounds, I think the G12M-70 would be a good compromise.
They have less mid-range and are more neutral sounding than most Celestions, but still retain some of the Celestion character.
It also depends on how you plan to use the speaker. The G12M-70 sounds fairly good clean, but not so great when pushed into breakup. They sound better with preamp gain to my ears.
Personally, I think any speaker you choose will be a compromise. In general, speakers voiced for Marshalls do not do Fender clean well, and vice versa.