Mortgage or RRSP - which one should be first?
The answer to this question depends on many factors, and varies from person to person. Ideally you should be able to do both. You can maximize your RRSP contribution, and then pay down the mortgage with the tax refund you received.

Should I borrow to contribute to the RRSP?
RRSP loan can be both a good or bad idea.
The good side of the story is that an RRSP loan generates forced saving, immediate tax savings, and tax-deferral growth. The downside is that interest expense could eat up a lot of your return if the loan is not paid back in short term. Interest charge is also not deductible for tax purpose. ake out a loan if you don't have the cash.
The best strategy is to borrow a loan to max your RRSP contribution, use the tax refund to pay down part of the loan, and pay back the entire loan within one year.

Can I hold U.S. dollar denominated investment in my RRSP?
Generally, you cannot hold U.S. dollar investments in an RRSP. By law, only Canadian currency can be held in a registered plan. However, you are allowed to hold U.S.-dollar denomiated securities in your plan, including the U.S.-dollar units of mututal funds. The downside for this is that you have to pay currency conversion costs to convert Canadian currency to U.S. dollar to purchase the security, and pay one more currency conversion fee when you sell the security.

Understand withholding tax
When you put money into your RRSP, you get a tax break. But when you take money out, you have to pay a withholding tax. Any withdrawals under $5,000 are subject to a withholding tax of 10% (figures are for all provinces except Quebec). So if you take out $4,000 from your RRSP, you will only get $3,600 because $400 (or 10%) will be sent to the Canada Revenue Agency for taxes withheld at source. A withdrawal between $5,000 and $15,000, is subject to a 20% withholding tax and any withdrawal greater than $15,000 is subject to a 30% withholding tax.

Note that the withholding tax rate is not your marginal tax rate. Often people try to minimize withholding tax, thinking it will be their total tax bill, and they are then surprised at the end of the year when they have additional taxes owing.

In-kind contributions
In most cases, people contribute to their RRSPs with cash. But if you are holding non-RRSP investments, such as Canada Savings Bonds, bonds, mutual funds or stocks, you can transfer them to your RRSP in kind or ‘as is.’  This can be a less painful way of getting an RRSP deduction than having to come up with the cash or take out a loan. Just remember that when you transfer the investment into an RRSP, it’s considered a disposition and there may be capital gains if the value has gone up.