Which project would be better : X-Y Position Control System using PLC or Temp. Control system using SCADA?

plc

In my opinion the project with subject X-Y position control system is more intresting than a scada system.

Having dealt with a variety of systems in robotics, machinery and processes, they can both have their attractions. I do like moving machinery as you get to see positive results when you get the thing to move properly. Beware though, because you can do a lot of damage moving a machine about.
If you go with the X-Y positioning system (what is wrong with adding a Z as well?) remember to give a thought to the mechanical dynamics of the system. You might also get interested in synchronising motor operation to ensure both ends of the beam move as though connected by a single shaft (if you use two motors instead of just one).
In the temperature control scenario the more interesting thing would be to use a fluid flow as a thermal carrier agent and get to control several heaters to achieve a specified temperature gradient (or series of gradients) along a product pipe-line. With this the earlier heaters will affect the later sections so the control loops get quite interesting.