Though it'll delay your progress at first, upgrading your trees is well worth it - they'll yield fruit quicker, and thus you'll start to earn more Pips from each visit.
Don't bother with décor unless an animal specifically asks for it. In the early stages at least, it's just a waste of money.
To work out which fruit trees are best, figure out how frequently you're likely to want to check in. There's no point in planting several apple trees if you're not going to keep the game on for very long.