Is the difference between i5 and i7 something you would factor in?
No. Typically the main differences between Intel's i5 and i7 are that the i7 utilizes Intel's implementation of simultaneous multi-threading and has more L3 cache. The difference between single models is mostly base clock speed. Simultaneous multi-threading (Intel call their technology 'hyperthreading') is what sometimes is described as "one CPU core acts as two" in layman's terms. A thread is a unit of CPU utilization but in order to handle/define such a thread not all components of a processing unit, or of a physical 'core' (including actual computation units), are necessary. So it is sufficient to double certain components in a physical CPU core but share others, including caches and executions units, to allow a processor to issue instructions from multiple threads with every CPU cycle.
Due to costs and limitations, you obviously don't get twice the speed from an Intel Dual-Core with hyperthreading compared to a similar Dual-Core without. It's a form of parallelization but it's not the equivalent to actually doubling physical cores. How much of an improvement SMT can offer strongly depends on what application you're looking at.
When it comes to mobile CPUs in laptops it's a different story because some i5s also utilize hyperthreading. An i5 in a laptop is either a dual-core with hyperthreading or a quad-core without hyperthreading, an i7 is either a dual-core with hyperthreading or a quad-core with hyperthreading.
So between an i5 with hyperthreading and an i7 with hyperthreading, the only differences are L3 cache and clock speed, depending on what models you're actually comparing.
When you compare dual-cores with lower clock speed to quad-cores with higher clock speed, the quad-core will obviously win, when you look at a dual-core with higher clock speed vs a quad-core with lower clock speed, it might again depend on the application you're running.
In the case of Lenovo's T570, all available i5 CPUs support hyperthreading and none of the available i7 CPUs is a quad-core,
so you're comparing dual-cores with hyperthreading to dual-cores with hyperthreading. It's most likely not worth the extra $$$ at all. According to the Lenovo website, the upgrade from an Intel Core i5-7300U (2x 2.6, 3MB Cache) to an Intel Core i7-7600U (2x2.8, 4MB cache) costs $170. You can buy an SSD plus additional RAM for the same money and you're good to go.
Needless to say that you won't notice any difference in everyday's life (browsing, office applications, text editing, Skype, photo editing etc) whatsoever.
Hope that helps.