On the surface, there are all of the user interface differences between both the graphical portions and the input methods.  Naturally, the iPhone is much better on both of these.  The LG Muziq doesn't have a keyboard and doesn't implement the HID profile for Bluetooth, so you're stuck with the 12-key keypad.  It does quite a good job at trying to make the most of it, though.  However, getting used to the interface takes some time because it is different from most.  For instance, it does things like click-and-hold for certain menus to come up on certain items.   Once you do get used to it, though, it's actually quite functional.

However, feature-wise, there are some differences and you might -- or might not -- be surprised at which one I think is actually better and why. 

Service-wise, the iPhone supports Yahoo! Mail, GMail, .Mac, AOL, as well as IMAP, POP, and Exchange (although, I think this is just an IMAP setting as it warns that IMAP must be turned on).  The LG Muziq client, on the other hand, supports AOL Mail, AIM Mail, Hotmail, Yahoo!, Gmail, PCS Mail (Sprint's own, I guess?), in addition to IMAP and POP.  The primary difference you'll note here is that the Sprint phone lists Hotmail.

Another interesting difference you'll find even before you receive your first email on the Sprint device is that it supports some form of push mail.  It warns that with "push turned off, you must select Send/Receive to retrieve new messages from the server."  After turning it on, it warns that push notifications are charged at the text message rate. This must mean they are doing something like J2ME-directed messages (I forget which JSR that is).  The iPhone, on the other hand, can be set to check for new email every 15 minutes, 30 minutes, 1 hour, or just manually.  The difference here really depends on what push means and how often it can do push.  Are their servers really checking email any more frequently than the iPhone itself is checking for email?  The answer appears to be a yes, which is interesting -- that's a fair amount of traffic given enough users.

Finally, the feature that I find gives the edge to one of the clients. You'll quickly find that the iPhone client works just like any typical desktop client for checking email.  It manages it's own interpretation of what has been read or not.  If you delete something, it goes in to a local deleted items. Messages are left on the server so a desktop client can check them.  In short, it's not unlike if you were using POP from a couple of different computers, even if you're using Gmail or whatnot.  In fact, I think the way it does Gmail is actually through POP.  On the other hand, you've got the client on the LG Muziq that handles the web clients a bit differently.  It is much more of a sync operation.  So, if you've read an item on the Sprint handset, it'll be marked read in Gmail or Hotmail.  If you delete an email, it'll also be moved in to the deleted folder on those services.  This may or may not be what you really want, but it does do that.  What this means, though, is that when you get back to your browser and check your email there, you can see which you've read on the Sprint handset but that's not the case with the iPhone.  If the client also supported Gmail tags and Hotmail folders for moving items, it would be nearly perfect.

Why not perfect?  Well, there's one other feature missing that is of limited importance for typical emails but useful with mailing lists and newsletters.  That's the ability to read HTML mail.  It only reads mail in text format.  The iPhone client is far superior here.  It can read HTML mail, as well as most document format attachments including PDFs and Word.

Unfortunately, the lack of syncing the read status has been quite a nuisance.  So, in the end, the client on the LG Muziq is actually better from a purely feature point of view and primarily because of the push and sync nature of the client.  The user interface and usability of the iPhone client, however, is going to be hard to beat by any non-smartphone client.