Re: Battle of the Marshalls...
Here's my opinion for what it's worth.
The JCM900 series IMO was Marshall's first venture into the truly high gain amp arena. As a result, it has a different tone stack more diode clipping (I believe) and more pre-amp tubes driving the tone than some of the more classic Marshalls. Pro's and cons? More gain, more of a "modern sound", probably more flexability and versatility than other Marshalls. Thesse amps seem to elicit a strong respones....people either love them or hate them...not much inbetween. I hate to pidgeon hole amps into a genre, but this amp seems to be geared towards the modern rock/nu metal group.
DSL 50. Marshalls attempt to meld the older Marshall tone with the more modern sounds. In some people's opinon, the best amp they've produced since the Plexi or the JCM 800. It's got more versatility IMO than the 900's, but I'm a fan of the Plexi and JCM 800 tones, and the DSL does those better. It has an arguably better clean channel than the 900, but that's pretty subjective. It's got all the gain on tap as the 900's but also backs down better to the classic rock/dirty blues level IMO.
If I was going to choose between the 2, I'd go DSL 50. More of an all around amp. Both can sound fantastic, just depends on your taste and style.