I'd suggest taking all of them
That adds up of course but they all offer different views into your ancestry.
If you are wanting deep ancestry, as in 1k-20k+ years then nothing beats the Y DNA test. Others have included a lot of info so I won't replicate it, I'll just add that as others have described Y DNA is what you inherit as a male from your father, and their father, and their father etc... mtdna everyone inherits from their mothers line very similarly. What makes Y DNA so useful for both deep ancestry and genealogy is it mutates more often than mtDNA. What this means is that each son that inherits a copy of their father's DNA has a chance to mutate some of the common markers passed down, that is then passed down to their sons, and at some point it may mutate more.
This allows you to compare the Y DNA of multiple males and guess how far back they share a male ancestor, if you share the surname and are a close match it may suggest as close as 8 generations or less... if not maybe 20-40+ generations. Since it mutates more often than mtDNA it allows us to narrow down how far back these mutations go back due to a higher rate of change.
This also allows Y DNA haplogroup research to progress at a much faster rate since there are a lot of markers they are still discovering that create sub-branches of haplogroups. Each year many new sub-branches of sub-branches are discovered... all of them narrowing down the time to a common ancestor for that group. For example here's the base tree of haplogroups:
http://www.yfull.com/tree/
Then you can look at the R1b one for a lot of sub-branches:
http://www.yfull.com/tree/R1b/
New branches are discovered each year... a parent branch may be estimated to a shared paternal ancestor say 5,000 years ago, then a sub-branch at 4,800 years ago (the point at which the new marker(s) mutated in a paternal ancestor), then a sub-branch of that sub-branch 4,500 years ago etc.
Much research not only estimates the age of when the first male with that marker mutated, but also geographical regions of origin.
For example my full breakdown currently is:
R1b > R1b1 > L389 > P297 > P269 > L23 > L51 > L151 > P312 > U152 > L2 > Z49 > Z142
U152, about 4 steps up from the bottom of that most likely originated in Northern Italy estimated around 4700 years ago or so, because of this it's called Italo-celtic since it most likely correlated with one or both of those cultures. Some believe that some who test with this marker with British ancestry might have come through Romans to Britain, though most of the movements into Britain most likely came much earlier by Celtic peoples (often guessed as either Gauls or Bell Beaker).
Z142 has an estimated age of 4500 years ago. That means it's estimated all men who are in this group share a maternal ancestor 4500 years ago.
Right now under Z142 there are two branches and a "*" branch with two people in it, the * branches indicate people who have tested negative for the markers of the existing branch. Basically people who probably are in a yet undiscovered branch, there just hasn't been enough markers amongst enough people to determine it. I'm probably part of a new branch, since another person who shares some distinct markers with me has already been tested enough to prove he doesn't belong to another branch. Right now I'm waiting for my "Big Y" test results from Y DNA and hopefully if I match we'll be able to discover markers for a new branch (or at least be one step closer).
That new branch will narrow down the age of our ancestors it also gives more data points for establishing geographical origins that correspond to those markers.
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