You shuold buy 32nm since its has best overclocking potential you can push it over 4Ghz using nice cooling by the way which are your option i can tell breifly if you mention then still 45nm's also not that BAD..
In Core i3 (all are 32 nm) best is Core i3 540 @ 3.06 Ghz
In Core i5 (all are 32 nm other than Core i5 750) 661 is best is you want if you only want to use IGD (integrted graphics) since it has 900 mhz GPU clock and if you wanna use discrete sol. then use core i5 650 (32nm) fastest 32nm is Core i7-980x
Remember AMD has not even single 32nm in market in 45nm u can consider the too
As a process technology of course the 32 nm is better, but in fact there is hardly any overlap between the two in current processors (so you don't really have the choice):
-If you want a dual core intel processor then the choice is clear: 32 nm core i3.
-If you want a quad core intel then all are 45 nm (and they are good).
-If you want a six core intel processor (and can pay for it), it is a 32 nm processor.
As khicharkumar said, list your options and your budget to help you choose.
For a 1156 platform depending on your needs the i3 530 is a good starter.
The next step would be an i5 750 or 655K after that an i7860 or i7 875K i wouldn't even consider an 870.
As for 1366 the i7 930 is the best choice.
And AMD has many viable options to choose from.
Like cobot pointed out it will depend on your computing needs.