Is the 3DS XL worth the purchase?

So it can be any 3DSXL and not just the pikachu edition?

Yep. Any model of 3DSXL counts, so long as you register it and one of the two games between the dates of 14th Feb 2013 and April 30th 2013.

The NoA website for the promotion even shows a Black 3DSXL in its advertising.
http://club2.nintendo.com/3ds-xl-promo/

Edit: That said, I suppose I should mention that There is probably a good chance nintendo will announce another (probably superficial) 3DS model at some point this year, because they've announced a new model of DS/3DS for the last 4 years running and I don't see the chain breaking this year.