It varies according to the each exchanges. I am not sure why they don't have the same value for the bitcoin or other altcoins in the marketplace. When I need to transfer the fund to anyone, I keep check the preev.com to confirm the bitcoin value and for altcoins I use coingecko.

I would say you can invest on the concern exchange where you find the high value. For example, I keep on using the localbitcoins to exchange my bitcoins for fiat cash because when compare with all the sites you can get the highest value there.