:welcome:Terry.
If you want to make a bit of money or be competitive in getting work, you need to find an adhesive that you like/is good/and fairly priced.
Both brands you have quoted are excellent adhesives - but there are others you may wish to consider. Click on the Mapei sponsor link to check out their range and find a local supplier.
Your later question about tiling over existing tiling and painted walls, will have a better response if you repost it in the
tiling forum.
You'll need to consider the weight aspect of the new tiles on both surfaces and the preparation that will be required for both.
I'd use your next 2 weeks of training to ask this question of your instructors and make notes to compare with those replies from the forum.
Remember as soon as you have enough posts you'll be invited into the 'Tilers Arms' and the world of Tilers Forum will be opened up to you.
So - Keep posting and I'll look forward to replying.