I think the Sheffield in the Peavey 6505+ Combo is pretty comparable, sound-quality-wise, to the Celestion Seventy 80 in the Marshall. Of course, they do sound different, and it is hard to do a direct comparison because these two amps are voiced so differently.
In a nutshell, to me, Sheffield have more of a dark, guttural sound where Celestions are typically more open sounding, and I think that about holds up with both of these speakers. The Sheffield works well with the 6505 sound (though Peavey seems to be moving to Celestions in their 6505 cabinets for some reason), and the Celestion is a good choice for the Marshall DSL, in my opinion.
I've read a lot of reviews from people who say they love these amps, but the first thing they did was swap out of the speaker. While there is nothing wrong with experimenting with different speakers, I really don’t think that’s necessary for most players. Both amps sound great as-is, and I have no intention of changing out the speaker in either.
Also, be aware that the new Marshall DSL40CR has a different speaker than the DSL40C. The new version comes with a Celestion V-Type speaker, which some players seem to like. When I checked out the DSL40CR, I thought it was pretty comparable to the old model at low and moderate volumes, but I did like the addition of the Master volume control.