Depends on what programmes are you going to run.
You can save a lot with cheaper tablets if Atom and 2 to 4 GB RAM are sufficant for your use.
Otherwise you have to go with something as expensive as the SP3 series, in return for not only mobility but also performance.
Prices vary with CPU (i3U-i7U, Core M), RAM (4-8 GB) and SSD (64-512 GB), etc. Only you youself know the spec you need.
(On the other hand if you drop mobility, the Ultrabooks are fast and more affordable than high-end tablets.)
The simple answer is that when under heavy load, the Core i5 runs at higher temperatures than the Intel Core i3, and in order to prevent overheating the chipset gets underclocked, resulting in slightly weaker performance in certain situations. In other words, some of that extra power that you’re virtually paying for can sometimes go to waste, allowing the Core i3 to run cooler, more stable, and to eventually take the lead.