I still ride a 2S, but if I could afford it I would buy a new CRF250 and a new RM250. As I'm now older and not so fit as I was I would probably use the CRF to try and get fit enough to do the RM justice, but I'm pretty sure that wouldn't happen and the CRF would be most used. Most people I speak to who have gone 4S have done so simply because they are so much easier to ride and have fun on, and those that race have chosen 4S because of the unfair capacity advantage presently enjoyed as a result of the rule makers desire to promote 4S over 2S. As for "which is best" its clearly a matter of personal preference, but its obviously true that given a level playing field 2S would win all the races, and none of the racers would ride anything else.
One other point worthy of mention is that many people who had given up MX are now being attracted back by the easy to ride 4S, and of course now they are older and richer they can afford to buy a new one, and so it could be said that 4S are making the sport more popular for participants. As has been said before, the reverse is true for those not involved, and dirt riding is very unpopular amongst non-participants, largely because of the increase in perceived noise, and also because there are just more dirt bikes.
The arguments will continue for a while as yet, but it cannot be called a fair debate until the capacities are equal. As for the wonderful technology, it is interesting to observe the fruitless efforts of 4S designers to attempt to overcome the basic technological advantage of an engine which produces a power stroke for every revolution rather than employing a complex arrangement of valves and reciprocating mass to produce half as many, thus meaning that the engine has to be spun faster to produce the same result. F1 cars are currently revving to 18-19000 rpm and have to employ all sorts of truly expensive materials, air operated valve opening etc., the point being that it is much more expensive to get power from a 4S due to its inherent disadvantages, and dirtbikes will need to employ similar systems in order to advance once the current advances in electronics have matured.
FWIW I remember being very upset when the "fart-box" 2S started to dominate "scrambling" and the marvellous noise of 500 BSA with a megaphone exhaust slowly dwindled. If only they had thought back then of limiting the 2S to say two thirds of the capacity of "proper" bikes, we'd have much less to talk about now. :clue: