There always have been a few hundred dollars difference in price between 13" and 15" - why would it be stupid to get the 13" because it is only 200-300 dollars less than the 15"? If the mobility is your priority and if you have an external screen at home, get the smaller display. If you need the 15" and you accept carrying the bigger one around, then get that one (15" isn't actually that much heavier, but it's bigger and wouldn't even fit in the bag I use for my daily commute).
At least with Apple you get 16:10 displays, they are the only ones still offering them and thus a 13" Macbook has a bigger screen than any other 13" laptop. I have a 13" and a 15" next to me and I honestly don't care about the difference so much, I can set the smaller one to the same high resolution of the bigger one. So the same content fits, it's just all a bit smaller.
The only way to save money is to get a model in a store instead of buying it from Apple. It's usually 150-400 dollars off the original Apple pricing, a few months after the release of a new model. Right now the prices for 2017 models are starting to drop. The only issue is that not all configurations are always available.
For the trackpad you should install (on any Macbook) BetterTouchTool, it allows some tweaking and for me it makes the big touchpads almost as easy to use as the smaller older ones. But yes, the 15" one is ridiculously sized, while the 13" is perfect. At least with the new ones I can click anywhere with the same required force and the button can't get stuck as it happened with my older Mac. Everything has tradeoffs and overall I prefer the newer touchpads.
For speed difference, the 13" just lacks those two additional CPU cores. When I edit photos, I notice it a lot as both cores on the 13" will be maxed out and the Mac will slow down, the Adobe software will start hanging annoyingly for seconds, and depending on the task even minutes at a time. On the 15" the CPU is rarely fully maxed out under those conditions and I will barely notice any slowdowns.
Once the 2018 MacbookPros with 4 CPU cores in the 13" model (and 6 cores in the 15") are released, I will switch to a 2018 Mac, definitely.
But the single core performance is amazing, half of my applications don't use multiple cores and that half is much quicker in everything. (And the multicore half is slower. So it goes.)
Can't say anything about the speakers, only my 13" is a 2017 model. Those speakers are great, in any case. They are among the best I have ever heard on a laptop, especially for the small 13" size. And Apple used to have about the worst speakers.
