If I am not mistaken - the ACE 150 does not have the pinpointer mode that the ACE250 has.
It is worth every penny.
I got a MD for Christmas - never told my hubby what I wanted (I had no idea anyway) and he got me the Discovery 1100. I think it was around $100. After 1 month I knew I needed a better machine because I loved the hobby. I bought the ACE250 in Feb which is a fantastic machine for the price. So basically, if you KNOW you love the hobby then try to save a little more and go for the new ACE250. If you aren't sure, then you might want to try to borrow one from someone and see how you like it first.
However.... this hobby is VERY addicting!