I recently bought Rugby Challenge because I was in the same position as you. Rugby 08, though fun, was getting far too easy.
What I would say about Rugby Challenge is:
- It is pretty much the same game as WCR with a few minor additions and animations.
- The goal kicking system has been changed, though I prefer Rugby 08's technique
- There is virtually no kicking game. The punting, drop goals and bombs are superficial at best and have no real tactical use. If you're a tactician like me and love the terrirory game then Rugby 08 is far superior.
- Running rugby is the name of the game, but yet again as with WCR it is whoever is faster at tapping X wins the rucks. This is my major gripe with the modern rugby games; the rucks and mauls are awful. No control or power over the outcome which shows the people making the games weren't big rugby fans because otherwise they would have cared more about this area. Thats why JLR was good. Passionate fans in a small design team making a game they cared about. Sadly small game firms have been beaten into submission by bigger companies such as EA who care more about profit than the end product. Hell, its taken then nearly 20 years to make a Fifa game that is better than PES.
- Classic matches are pretty boring to be honest and only add slightly to the gameplay value.
To come back to your question, I think Rugby Challenge is harder than Rugby 08 only because if you are a minnow, your players are ridiculously slow and get smashed off the ball every time. That plus a shoddy kicking game means its more frustrating than anything. There is an arguement that the multiplayer is fun but as I said, once you find a friend with a faster finger than you, you're screwed. Rugby 08, once mastered, can become repetitive.
I found a copy for 4 quid so its not a massive investment but don't expect a hidden gem. Heres hoping that EA come out with a brilliant new rugby game soon!