When it comes to the user interface (UI), I have to say I prefer Hotmail, because I play lots of PC games, I have to use Microsoft Windows, the Hotmail interface then seems familiar to me since I am familiar with Windows 7 which has a similar UI, however, You could say that if Hotmail is you average sedan, then Gmail is your Hummer H3, Gmail is packed with features for corporate users and home users alike, Gmail allows user to download their e-mail in their favourite e-mail client using IMAP or POP, this is certainly a plus in my Book.
The new version of Hotmail also boasts Active View, it allows you to embed content in an e-mail, for example if you "embed" a Youtube video then, it would look like this,
instead of this: http://www.youtube.com/watch?v=qAdwK_biEMQ
Basically it allows you to view certain items in your browser, without needing to open a new tab/window.
HOWEVER! (GMAIL CREDITS)
On the other hand Gmail constantly rises to the occasion in terms of speed and reliability, for example after you send a message, it tells you the message has been sent, in Hotmail it wants to know if you want to add this person to your address book. Sure it makes sending e-mail to the same person about a second quicker than having to type the e-mail address in.
If Gmail didn't have a proper search function I would be worried about the future of Google, the search function in Gmail is quick and concise, (I am pretty sure that is what made Google famous :p)
Gmail also boasts Instant Messaging, but before all the trolls appear form under the bridge to tell me Hotmail has it too, who had it first?
Gmail ability to allow you to organise your mail is excellent and couple it with the ability to view your e-mail on almost any phone is quite impressive, where as Hotmail mainly focuses on the I-Phone.
Both sides come up equal in my eyes in terms of Spam filtering, I don't much Spam so I can't speak too much about this.
However if I were to recommend a service I'd tell you to use Yahoo
On a serious note, I would like to give Hotmail a fair try, but since I am glued to using my e-mail client, I would stick to Gmail!