What is better? For-loops or generator expressions (in cases like this)
The second version can (and should) be written using sum instead of reduce(add,...), which is somewhat more readable:

sum(i for i in xrange( 1000 ) if i % 3 == 0 or i % 5 == 0)


That is the version that I'd prefer as far as implementations of this particular algorithm go because it's more declarative than a loop and just reads nicer in my opinion. (It's also faster though that's less of a concern).

However it should be noted that for summing all numbers up to n that divisible by 3 or 5, there's actually a better algorithm than going over all those numbers and summing them.