in this case it comes down to a few things

1) the games you play - do you need all the buttons in the G300? or will the simple setup of the spawn is enough?
2) Button placement - is having a side button a must?
3) mouse type - ambi or ergo?
4) mouse surface - plasticky ok with you or rubber feel?

answering these should be able to help you to decide at least I hope it would

what game do u play?

I own g300... at 1st, its button placement.. its "side" button actually not on the side on the mouse..if b4 u used to side button, u need to take some time to use to g300.. 
& its quite light compared to other gaming grade mouse.  &, its scroll not really has "clicky" feel... which not used to it at 1st when scroll while browsing on9.
    btw, that extra button makes sense in certain software.. think about its possibility.  logitech gaming software interface for this mouse quite user-friendly

a bit weird at 1st...but now im used to it.   loves this g300.    espcially the extra buttons. 

i was also considering here and there when choosing a mouse, but d700 came to my attention because of the good reviews everywhere (literally), and the wireless and rechargeable is an awesome plus, all the freaking buttons on it is a compliment

g700 expensive lou~ but wire+wireless , rechargable and many programmable keys, for rm2XX is worth it, and i eager to go home open and use it

IMHO for the corsair M60 you're paying more to the brand and aluminium base.. feature-wise its almost the same with most mice in the market.. so its kinda like overpriced for me... IINM its priced the same with a G700?? correct me if I'm mistaken abt the price 

well after a night using the g700, the sandy surface is not bad, my hands dont feel sweaty as much now, dont know what sorcery is that, profiling is swift, buttons are kinda hard, but it confirms you that u pressed it, wireless quite lasting, charging cable is so damm heavy.. my god, drag on cable feels like the cable got tangled with other cables, when it's not, battery indicator and profile indicator is very useful, scroll wheel is so silky it makes me nervous dunno if i overscroll or not with such smoothness,

software is as they say, ugly but straightforward, easy to understand all the things inside, i depend on their preset profiles, and is very well configured

one more weird thing, i took the liberty of shaking the mouse to see how sturdy it is, lol got rattling sound coming from inside, think from scroll wheel, weight on cable is like a tonne, but detach cable becomes featherweight