You can get better exchange rates if you buy directly from an exchange like Bitstamp, Kraken and Coinbase. If you don't care at all about getting the best price, using Xapo would obviously be the easiest way (since you already have an verified account). It all depends on what you are looking for.
I have already registered on Bitstamp, Coinmama and some other exchanges too. But, they took a long to verify. I submitted documents two weeks ago. But, still Bitstamp/Coinmama verification processes are pending. How long it will take?
-if you eye for long term...
-right now that you are currently having a problem on verification at the mentioned exchanges for weeks...
-you really need to buy bitcoin right now...
-you are fine with XAPO fees...
I don't see any problem if you used XAPO to buy bitcoins. All exchanges are good for buying, it's just that they have different fees. But since you are a verified XAPO user, you are now one click away for buying bitcoin at them so you can consider it now rather than waiting for long period of waiting for your verification on other exchanges on which some of them takes months.