Big Xcode compiles. Anything that spends most of its time reading/writing lots of files.
No. The source files are very small, relatively speaking, and Xcode caches them very efficiently. Compiling code requires CPU power and RAM. HDD speed is less important.
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Here's what I mainly do:
-Safari/Chrome
-Mail
-iTunes
-Vuze
-Handbrake
-iPhoto (I have a Canon 60D that Im investing in as a hobby and like to spend time with so I also edit very lightly in photoshop, etc.)
-Light gaming (Xplane, possibly looking at using it for Diablo III) HOWEVER, this isn't too much of a concern as I have a Xbox 360 I mainly use if I want to game)
The big deciding factor is that whatever model I choose, it'll be my only computer. I have no desktop what so ever. So which ever model I choose that will be my only and main computer. I go to college as a Civil Engineering major, but don't depend on my laptop for anything other than writing documents etc. and I also never really have a need to take my laptop with me anywhere so it'll spend most of its time sitting on my desk and moving to a chair stuff like that.
If those are the things that you mainly do, then you can easily stick to a HDD and get a 17" MBP, even though it will be even faster and snappier if you install an SSD. The SSD makes a huge difference in file I/O. Simply surfing the internet does not have much to do with the hard drive so I am not sure why you're wondering why surfing the internet isn't any faster with an SSD. It's like asking why you can't watch a DVD faster on a 48X DVD drive versus a 10X DVD drive. Because watching a DVD drive does not require the drive to be reading at 10X or 48X.
Honestly, you could have just done 20 minutes of research and found out a little bit about how computers work, but I guess that's just too much effort for people today.
Here are some "real world" benchmarks to show you the differences between an SSD and a mechanical HDD.
An SSD won't help you encoding in Handbrake because encoding is CPU bound. A hard drive is fast enough
However, what about resizing 140 image and creating thumbnails automatically.. something that people mucking about with photography might need to do. There is CPU involved and RAM and the speed of your hard drive. The 4 bottom bars are all done on the same 2011 Mini with 4 different configurations. The SSD makes a huge difference. Despite the fact that the MBA has a slower CPU and a "slower" SSD than the Mini, which has a Vertex 3, the real world difference is only a couple of seconds.
What about resizing 140 images and then watermarking them via a pixelmator automated service? Well, this is very much a RAM bound operation. The CPU does not make a huge difference. It takes 980 seconds to do this on a stock 2011 Mini Server with a 7200 RPM hard drive and 4 GB of RAM. The 2011 MBA manages to finish this faster even though the Mini Server has a much more powerful Quad Core processor. This is because of the SSD in the MBA. However, things change when we add another 4 GB of RAM to the Mini. The time to do this is now 287 seconds, which is a huge improvement, yet when we add the SSD as well, the time is 206 seconds.
Even the lowly, old 2009 MBP with a Core 2 Duo processor manages to beat the stock Mini Server quad core. That's because the 2009 MBP has 8 GB of RAM and a Seagate Momentus XT hybrid HDD.
Hopefully I haven't wasted time typing this out and you've actually learnt something.