What limitations are you experiencing with the 3100. if there are non you have your answer. Does the 7100 solve those limitations?
Good question i asked myself many times
first thing is the higher mega pixel which i have no use of but want to
second thing is the hdr picture capability
3rd thing is higher iso capabilities which would let me take pics in darker environments
most importantly i wanted two features one is the remote to take pics (just saw this is another $200 )
and another feature i missed in D3100 is (i forgot the name of that feature but) the camera actually takes like 3 shots instead of one at + and - fstops .. like the fstop is f2.0 then the pic gets taken at f1.8 and f2.0 and f2.2 and we can choose while we edit.
What are you doing in the dark with the 7100? Are you vampire? He He
Good question i asked myself many times
first thing is the higher mega pixel which i have no use of but want to
second thing is the hdr picture capability
3rd thing is higher iso capabilities which would let me take pics in darker environments
most importantly i wanted two features one is the remote to take pics (just saw this is another $200 )
and another feature i missed in D3100 is (i forgot the name of that feature but) the camera actually takes like 3 shots instead of one at + and - fstops .. like the fstop is f2.0 then the pic gets taken at f1.8 and f2.0 and f2.2 and we can choose while we edit.
A $200 shutter release remote?
http://www.bhphotovideo.com/bnh/controller/home?O=&sku=207373&Q=&is=USA&A=details
You can bracket with any DSLR (although the 7100 makes it a bit easier) Lightroom, Photoshop, and I think the Nikon software will allow you to compile the images. Because of harsh lighting and moving objects I have found bracketing extremely difficult in aquarium photography (not sure if that is your intent).
Something else to think about... quality/professional glass will also provide sharper images, better color rendition and typically a few extra stops of light. Are you happy with your current lens? If not, you will have a much larger photo with the imperfections of the glass.
All that said the 7100 is a great camera!