Toyota Repair: Timing Belt, timing, belt

Question
Vince, My 1st question is from curiosity: why did auto
makers change from timing chains to belts? It seems chains
are more reliable. My 2nd question:  My 2003 Sienna has
120,000 miles. I guess it's overdue for a timing belt.
Should I expect to have the water pump changed too or might
that not be necessary? I've heard vehicle also sometime leak
oil unless the mechanic replaces various seals and gaskets?
Is that always necessary?  If so, is it usually included in
the cost when we're quoted a price for a timing belt? A
local mechanic in Houston says they'll change the belt for
$300. Another mechanic who is usually cheaper says $600. I'm
assuming the difference is in what all they do in the
process. I figured I'd get an expert opinion before I start
questioning them. Thanks for your time, Vince.

Answer
Good questions!
Automakers went to belts versus chains for a number of reasons; economics, noise, and chains tended to stretch and start eating into the surrounding metal.
You should have your timing belt changed every 100,000 miles.  Also, change out the water pump while you are at it because they have to take off the same stuff to change a water pump as they do to change the timing belt, so you save the labor cost of doing it 10,000 miles later; your water pump will last the length of your timing belt.
They will probably replace your front main seal; again, they have the front of the engine disassembled already.  They'll need to replace any gaskets on parts they disassembled.  Ask for the old parts (e.g the belt, the water pump, etc) to keep them honest.
$300 for a timing belt sounds awful low.  Find out exactly what that covers, you do not want any surprises!

Hope this helps,
Vince