Neither is easier than the other, in and of itself. You can find simple kits in either genre.
If you'd like to build a simple aircraft, I recommend the 1/48 kits from Monogram, which were also lated sold under the Revell-Monogram merged label. Kits like the P-51D, the F4U, the SBD, or the P-40B, date back to the late 50s and early 60s. By today's standards, they are simple kits, but that makes them good kits for learning on. I still build old Monogram kits for fun, too, because I enjoy them.
As for armor, I will recommend Monogram again (also sold today under the Revell-Monogram label). Again, the kits are simpler than today's kits from Dragon or Trumpeter, among others, with parts counts approaching four figures. Monogram's armor kits are relatively simple, with low parts counts, but they build into nice-looking models, and they are good practice for someone starting out.
And to show how good a model can be made from either a Monogram airplane or armor kit, have a look here:
http://sheperdpaine.atspace.com/
Hope that helps!
Best regards,
Brad