For those who mainly like the thrill of the hunt of the base and chase, it is hands down Beginnings. It takes at least a case to put all of the low chase together and a great hunt to get the autos and holograms. Sketches are not the primary focus here, but you will tend to see more done by relevant Marvel Comics artists.
While there will be some beautiful sketches in Bronze Age (you will get shut out on Gambit as previously mentioned), there are 2-3 dealers who will open TONS of the product. One in particular is promising to have "over 2,400" sketches available. The mega busters seem to all get "above odds" on all of the best cards as well.
As far as "down the road value" both sets will decrease in time but it happens much quicker on the RA products as those same mega busters will dump all of the base/chase/master sets in order to move on to the next thing. There really are not any large dealers busting Beginnings because UD does not give them a boatload of incentives (free merchandise to lower the cost in volume) as RA does.