Google Drive, Dropbox, SkyDrive, Cloud drive and iCloud Comparison

Free Space

1. SkyDrive - 7G (25G if you are old user)
2. Google Drive - 5G
3. Cloud Drive  - 5G
4. iCloud - 5G
5. Dropbox  - 2G

SkyDrive is the winner by providing 7G free space. You can get 25G if you register with SkyDrive before. But you need to following the instruction and this offer is only provided for limited time.
Google Drive provides 5G but this is additonal to Gmail (10G) and Picasa (1G). Also, Google Docs and files converted to Google Docs format don't take up Google Drive's 5G space.
https://www.google.com/settings/storage/?hl=en
If add all the free spaces provided by Google, it is more than 16G.
Cloud Drive's 5G does not count the space used by the songs you purchased from Amazon.
Dropbox can give you 500MB space for each friends you referred (https://www.dropbox.com/referrals).

Paid Space

1. SkyDrive - 20G for $0.83/month, 50G for $2.08/month, 100G for $4.17/month
2. Google Drive - 25G for $2.49/month, 100G for $4.99/month
3. Cloud Drive - 20G for $1.67/month, 100G for $8.33/month
4. iCloud  - 20G for $3.33/month, 50G for $8.33/month
5. Dropbox - 50G for $9.99/month, 100G for $19.99/month

Although the free spaces may satisfy most people's needs, you may still want to get more paid space.  SkyDrive offers cheapest rate, only $4.17/month for 100G, while Dropbox's price for the same space is almost 4 times than that.
Regarding the maximum space, Google can provide up to 16T. Dropbox offers 1TB+. Cloud Drive is 1T.  SkyDrive only offers 100G maximum.

Web Interface
Dropbox
Dropbox's web interface is clean and simple to use.

Besides the basic functions (Upload, Delete, Search, Manage files and folders), it also have several interesting features.

• Sharing - You can share your folder to other Dropbox users. The shared folder will appear in their Dropbox folders. Shared users can add or delete files in this folder. The changes syncs to all shared user's computers.
• Links - You can click on the Get link icon of a folder or file and copy the link from the browser's address bar. Then you can send the link to users you want to share. 

• Events - Every changes you made to your Dropbox will have a log. This is very useful for shared folders. 

Google Drive
If you use Google Docs, you will found the Web Interface is very similar. Actually, Google just upgrades the Google Docs and renames it as Google Drive. Now you can upload files of any types.

What is better than Dropbox:
You can create all kinds of Google Docs, like Documentation, Presentation and also edit them inside the Web Browser.

• Sharing - Google Drive allows shares both folders and files (Dropbox only allows sharing folder). You can specify the level of sharing: View, Comments, Edit. Shared users must have Google account to access the shared folders or files. The sharing is not integrated with Google+ yet. So you can not share to a Google+ Circle.

• Links - There is no explicit link function. When you share, a email with the link can be sent to the shared users. You can also copy the link from the address bar. But you need to log on to Google in order to view.