First, similar specs on the laptops you mention will not be cheaper. The Acer S7 you mention comes in a model that has an i5, 8GB RAM, and 128GB hard drive, and it's $1299. For the same money, you can get the SP3 with the same processor, same RAM, and double the hard drive space. You will have to pay extra for the keyboard, so maybe that cancels out the fact that the SP3 has twice the hard drive capacity. But for similar specs, you're getting a similar price.
The difference is in the pen and the form factor. Whether the pen matters to you is a personal choice, but having one is certainly better than not having one. The form factor is a clear choice -- with the Acer, you get a clamshell keyboard which is going to be more comfortable on your lap. With the SP3, you get a significantly lighter device (even including the keyboard) and an extra couple hours of battery life if Microsoft's 9-hour claim is credible.
Most people do not actually use the laptop in their lap much, at least not in a formal seated position. If you are one of the few who spend much time using it that way, then the clamshell will be more comfortable than a kickstand. For most of us, the laptop's primary use is as a portable device that we carry in a bag or backpack to a desk/table in a classroom, meeting, or hotel room. For that use, clamshell has no advantage over kickstand. However, having a device that is lighter and thinner (i.e., takes up less space in your bag) can be a major advantage.
I'm one of the many that use the laptop on the lap only rarely (and when I do, it's in my La-Z-Boy with my legs stretched out, so the kickstand works just as well in that position) and that does have to lug it around to meetings and travelling. The SP3 is a no-brainer for my use. I pre-ordered one last night. YMMV.