I'm a big fan of both also, but the HW's(Beeman's)win over the Diana's(RWS) IMHO, at least in most cases. I have two 48's(.177 & .22), a .22 34, and two 24's(.177 & .22), I also have a 77k in .22, an HW90(RX-1) .177, HW95(R9) .22, HW50s(R6) .177, and just recently an HW30s(R7) .177. I really like the 30s, and it's over all a very well made rifle, but I still favour the heft, feel, and size of my 24's. The Diana's may not be as refined as the HW's but they are still well made and accurate rifles. One area in construction that Diana has the upper hand is in the finishing of the mainsprings and fitting of guides, the HW springs IME need more attention to remove burs and also to have the ends smoothed out, aswell as having more 'twang' because of the greater tollerance between spring and guide. On the other hand the HW's have the better trigger, although the Diana triggers are great aswell after adjustment and break in. As for power levels I'm kind of in the middle of the road, I love the range the high power rifles give, but my absolute favorite rifle is still my HW50s, the weight, size, power(11fpe), and incredible accuracy are what makes it so great for me.
I can't help myself, I love them all for one reason or another.
Mulby