i have gone through the specs of both the phones..But cant decide what to take.I am a college student and want to use my phone for tutorial videos,ebooks,music,social networkig,messaging and much more.Obviously a good camera is quite expected.
they both have almost the same range.please suggest which one to take with some expert comments.:)

As per your requirement, will suggest you for Spice MI 410, great phone at great value. Ace have very much low spec compare to MI 410. As you you are college going student, Networking, Videos and Games would be on your priority and I think Spice MI 410 is capable to fulfill your needs.

Don't go for brand name which samsung have. Following are pro and cons

Pro for Spice MI 410
1 GHz Processor
512 RAM
2 GB inbuilt memory
Large Screen (for video and gaming)
200 MB GPU
HDMI Output
with 3 G you can enjoy downloading speed upto 14 MBPS
HD Video Recording Capability
Good Camera with 7 X Zoom
Front facing camera (as 2.3.4 updates available. we can use Gtalk for Video chat[not confirmed])
good resolution (480 X 800) (good for ebook, Online Reading, and Videos)
Good Community Support (Techenclave, Lowyat forum)
Tested worldwide with different names

Cons for MI 410
Spice as a Brand Name

about defy, it is also good and very much better choice than ACE. So if you are not looking for Spice MI 410 then go for Defy. ACE is really worst choice at that price.