on foot....Explorer, if you are looking for a larger area and plan to drive around several sites then you may be better off with the landranger...but you will need an explorer if you are searching out any sites not signposted by Eng Heritage!
I use the Outdoor Leisure series for Dartmoor as it 1:2500 or 2.5 inches to the mile. I would advise getting that size as it seems to cover everything. I am no sure which is covered by either of the maps you mentioned.
Given that I lead my partner straight onto a partridge shooting range looking for Nine Stones Close (it was the feathers and the spent cartidges that gave it away) using a Landranger 1:50,000, I'd suggest Explorer 1:25,000 Every time!
Ohhh, yes - the 25k (orange covers) ones are definitely the best for detail and actually finding things.
I remember me and Mrs G going round with a touring map with the whole of Cornwall on it many moons ago! Still - we DID, surprisingly, find Boskawen-Un, the Merry Maidens and Tregiffian with it..! Bloomin travesty, it were... ;o)
As Nine Stones Close is one of the sites I'm heading off to see next month, I know which map I'll be buying - feathers and cartridges don't appeal!! ;o)
Locate the area's sites online at very large scale (RCAHMS etc.) then plot the ones you are interested in on your map of choice. Also useful to make a rough sketch of where they are with regards to fields and roads at that scale where not obvious in print (always bearing in mind that on rare occasions minor sites can be shown on the wrong side of boundaries such as walls and fences).