I think the i5 with 6GB is better since the i5 is a faster CPU/Processor chip. If you just plan on using Windows Movie Maker or Windows Live Movie Maker to edit short videos like 30 minutes or less then 6GB could be good enough. You can also always replace or add more RAM if you need more in the future, but you can't replace a CPU easily, or at all if the CPU is soldiered/sticked on the motherboard of the CPU.
I would say go with the i5. The i3 is the chepest of the core i processors, and the i5 is many times used in some gaming setups as well.
I would recommend that you google the i5 and i3 versions though for performance numbers. A higher end i3 processor can beat a low to mid range i5 processor.
It is also worth checking out what video card/Graphics chip is on your laptop since a lot of newer programs like Opera 12.10 uses the videocard to render websites, and a faster video card can make some programs run faster, and improve gaming on a laptop.
I was wondering if can post links of the two for full specs. It also check the reviews of each because I find them helpful. You don't want bad eggs, I know my laptop can overheat when pushed too far high processing and graphics, probably because I play games on it.
They are both direct upgrades of my current laptop, HP G6 i3 3gb ram, running Ubuntu 12.10 starting to get fan trouble, its not all the time but I would like to get an upgrade.
K, Depends on what your requirements are, I would go for the I5 6gb. As other people have said you can always upgrade the memory if allowed in the future. If you need any other people feel free to ask.
Always go for the better CPU and i3 is not equivalent to the i5. That is like saying the i5 is equivalent to the i7 - they are different chips for a reason.
So, the i5 will be better, especially is the choice for gamers these days even over the i7.