[MENTION=150899]ishan99[/MENTION] all those number you stated in 24vs 16mp , color bit and all snapsort things doesnt event matter in reality
24mp means bigger file ...which also mean u will have lot bigger RAW file like 40MB or so...now for that u will need higher processing PC too..and a twice bigger HDD too
color bit you will never come to notice even if u get a D7100 vs D3100
now flip out is very useful for macros
HDR is useful for landscapes
[MENTION=125321]nac[/MENTION] for him 40mm 2.8 will be good ...it will take good macro 20mm is for 1:1 distance ...just take 1:2 from 40mm
my lens is a 1:2 and I never have issue ...and even if you get 1.8 lens its DOF is too less at that...we have to shoot at 2.8 most of time for portraits
other question MF vs AF on macro - yes for macro MF and back screen is much more useful...but you can not shoot a butterfly with a MF lens...its just too fast ...and its good to have AF too for other uses like portraits
sigma 70-300 is a 1:2 macro lens at 200mm-300mm range...soo that makes it too long on front...very small space to capture...and lastly not as sharp as a macro lens ...but its definitely usable