i wud say vertix family of board and processors are always better as its a perfect platform for pre synthesis and the post layout simulations match almost approximatly compared to ASICs.
There's really no simple answer for that... "Depends on your design" is the only answer that's always right.
Best thing to do is to check what you need in design (user IO, memory, embedded processing, LE's, PLL's,...) and demand a quote from both and go with the best price.
Maybe others will disagree, but I have never found a design that couldn't be done with both Xilinx and Altera.
It also depends on which Tool you are using and your application to be implemented. Virtex4 are better suited for DSP applications with dedicated DSP48 blocks and Xilinx bets they can give you a frequency of 500MHz with full pipelined versions.
If ur design is sensitive to the cost, I believe altera is good choice. By the way, the tools altera provides is more convenient to use, in my opinion. ^_^