As a spyder owner, I'm inclined to steer you away. I'd either recommend a PMI Pirahna, mainly because the quality of their products and customer service is better than Kingman's. Even better though, might be something a bit more high-end like a second hand bushmaster, impulse or other relatively simple (and cheapish) electro.
If you're heart is set on a spyder type marker then that's cool, but you might find that the cash you spend on upgrading it will only improove it so much!
To actually answer your question, I'd say the Xtra because it comes with a verticle ASA (Air Source Adaptor), which means you can more easily add an inline regulator.