Both services sync fairly well to Apple, so you need to consider the pros and cons of each as far as usability is concerned.
Also, there's absolutely no reason to only stick with one provider for everything. Sometimes integrating services from two different ones works better.
Here's my experience, however biased it may be:
Email - either Gmail or Hotmail are fine, and yahoo syncs fine too.
Calendar - I find Google calendar to be superior. It loads faster in browser, it has a search ability (something MS for whatever mysterious reason decided to omit), it has - in my experience - much more robust sync engine (MS seems to still be using some version of ActiveSync that is prone to getting out of whack every now and then). Also, more great 3rd party calendar apps like Pocket Informant or Calengoo work directly with GCal but rely on Apple calendar for Outlook.com sync (losing some functionality in process).
Tasks - I don't use either service but instead use Toodledo which is far superior to both, even in free version. I imported Toodledo feed into Google calendar as a subscribed cal, which worked out very nicely. I am also syncing it to Pocket Informant on iPad. If I had to choose between MS and Google I'd go with Google Tasks, because they show in browser version of calendar on due date, again a very basic feature that MS stupidly omitted.
Contacts - again, Google. MS has problems syncing contact photos.
Office - for a quick basic spreadsheet I use Google docs, for more involved stuff Office Suite Pro which I also use on my Android phone. It works well with MS Office docs, and is reasonably fast. And way cheaper than Office 365. Plus, it seamlessly integrates with Dropbox and Google Drive.
Cloud storage - while I have free accounts with all major providers, if I only had to use one, I'd go with Dropbox. Unlike Onedrive, it allows you to specify files to be accessible offline. And unlike Google Drive, it doesn't try to get you to convert the office files to Google Docs format. Plus, the syncing is fast and reliable. Onedrive is slower. As to the privacy issues - MS employees are known to be snooping around users' private files on Onedrive for "prohibited" content like nudity, so I don't think either MS or Google are better in that respect. If I don't want my data to be accessed by someone, I zip it inside an encrypted file. This way, only NSA can see it
With this setup, I am completely hardware independent and can switch to a different tablet or phone tomorrow without losing any functionality. Also, it is much less complicated than it sounds, as all these services integrate very well into all three major OS I'm using (Windows, iOS and Android).