Purchased a new air conditioning unit for rental property should I use an annual election ans should I deduct the full value or take a partial seletion?

That is entirely a personal choice. If you need the deduction this year, then expense it. If you plan on holding the property for a long while, you may want a deduction each year. If your income is over $150,000, you will not be able to take your losses, so it doesn't make sense in that instance to take the entire amount this year.