Pit bike or Big MX bike?

I'm 16 and would love to start MX (May sound stupid but i would love to do it) so should I get a Pit Bike or a full sized MX bike. For pit bikes, i like the look of the Super Stomp SS120 or the Demon X XLR120. I'm not sure if these are any good. Apparently the Stomp isn't very good but demon are? For full sized, i like the look of the Suzuki RM85 which looks and is supposed to be very good. Is this true. What's better for me with NO experience and I'm about 5'7 or, should i just forget it all together?