I suggest using Google to search for the information. You need to look up the roles and responsibilities for these positions.
For the PM you will find a lot of information on the responsibilities. Basically they are project wide.
For the Test Lead you will find most of the information here. Basically they are specific to an area of the project.
Importance is debateable. I have been on projects where the PM was useless and others where they were indespensible. I have been a Test Lead (some people have said the same things about me, typically by the PM's who didn't have a clue) and believe my position is important to the company. But typically it is the PM that wins out when it come to who to cut.
That's like asking the same question about the CFO and COO! The answer depends totally on point of view and is irrelevant! The best example is Sales vs. Production. If sales can't sell it there's no use to build it, but if it isn't built, sales can't sell it! It's the old chicken and egg puzzle which the smartest person in the world answered by saying that at some point, something came from the egg, that was almost a chicken. So the egg came first.
It's obvious that a company could do without both a TL and a PM, but if I were to chose to have one without the other, I might choose a PM, and then again the next day I might choose the TL.
In my experience a PM manages the project and a Test Lead works on the project - both are important in their own way.
However some companies think that anyone can be a PM and so would replace the PM by any manager/lead that is available - this usually results in an adverse effect on the whole project.
A Test Lead implies that there are also testers so a test lead can be removed and the testers can continue - often resulting in the testing being not as good as the Test Lead has the experience and skills to ensure a better test coverage etc. The loss of a Test Lead or tester can result in a product that may not work as well or fit the requirements.
The general answer to your question is that "it depends"
I was once told that the most important person on a project is the Project Manager. If you have no-one else on the project, you need a PM, because s/he is the person who will tell management that the project is now impossible.
Makes some sort of perverted sense if you think about it.