Adrian..personally..if ya gonna go macro,,,,go macro...i use the Tamron 90mm Di VC USD 2.8 lens..be sure its the one that focuses internally.. its ALOT less than the 28-300 ,spend that little more if you can....you'll get a SUPERB macro lens..it also makes for a great Portrait lens on the d5100 which i also have... its my Fave lens...well worth the money spent...Super sharp and upthere with the best.
Btwi'm only saying this if you go Macro and would use it for portraits...Obviously doest have wildlife reach of 28-300..but the price is definitely alot better... heres one or 2 of my own.
The great thing is you can get some kenko extension tubes at an ok price and even add that to your macro lens. gets close to 2:1 its plenty enuff... when you start getting to close you gonna have to look at focus stacking( depending on how you like your macro)
These are only with my macro lens(tamron) and flash on camera with a home made diffuser...seriously...you can see flash in its eyes!!! you wont get that with 28-300., Go dedicated Macro...you know you want too.
On a side note..if ya dont get close its no fun