Which computer should I get?

The i3 system is better.

It is much more energy efficient, newer technology makes the CPU at the same clock (For example if they were both 2.4Ghz) about 5% faster. The i3 has hyperthreading which can give good boost in performance. It also has much better graphics technology.

Well do you keep a lot of tabs open while browsing?
This is where the biggest difference will come up. CPU cores by nature can only do 1 task at a time. They do it so fast that we don't even notice really. Since we do so many things at once now, and leave so many things open its become needed to run more than 1 task at a time. Thats a big part of why we have dual core CPUs. Hyperthreading allows the CPU to do 2 things though. So your Dual core Pentium can only do two things, while the i3 can do four things.

If you keep like 8 tabs open, 3 word pads open, maybe a video, and skype you will start to see a lot more slow down with the pentium than you would with the i3.

Glad I could help
Hope you enjoy your laptop.

I wouldn't go for the Acer if its much more. The Acer's CPU is a low power model so the battery will probably last longer. Normally it would give lower performance because the CPU runs at 1.8Ghz. However, during most processes your system will idle and run at a lower speed regardless of what system. Also when you do push it, the Acer will overclock itself up to 2.5Ghz and at this point be the equal of the i3 in the Toshiba.

If they are the same price, just based off the information posted, I would get the Acer because it will most likely give better battery life with little to no drop in performance.

If the Acer is more than $20 more expensive, I would not bother because the best performance the Acer can give is equal to the i3, and thats after it raises its power consumption to increase the speed, giving you a negligible lower power consumption when you are running intense programs, and equal if not very slightly worse performance. While not using intense programs you will using reasonably less power with the same performance.

One other thing, I just mention as a forethought. IF you are comfortable working on computers or know someone who is, I might have a laptop CPU for sell that you could buy either of those computers most likely and put in for more performance at a low price. My laptop which works perfectly fine I am thinking of upgrading the CPU is the reason I mention it.

Idk if I would say big mistake, but it would be a mistake. Get the Toshiba.