Pick a benchmark that's representative of your computing needs. Look at relevant benchmark scores.
Recent Intel CPUs are differentiated by their GPUs and TDP moreso than clock speed or thread performance, which is probably why a brand new Haswell i3 is only just a bit faster than an original Nehalem i7 from all the way back in 2008.
If you want top-end per thread performance, you probably want an i5. If you want that and need more cores than a typical desktop, get an i7. You probably don't need to worry about anything else; even five year old desktop and laptop parts are going to be subjectively similar to new for anything but a narrow range of content creation, gaming or scientific applications (assuming similar amounts of RAM and disk subsystems, that is). Whatever CPU you buy will probably be good enough for the life of the other components in the computer.
It's certainly a hassle to compare between CPUs on differing device types (e.g. is a 15W ULV i7 faster than a four year old 45W mobile i3?) but the truth is that within broad categories, newer things are faster and the classifications hold up. If you're doing an apples to oranges comparison, you have to look at whatever benchmark you think might be most relevant.