(Just noticed: Gradient's post passed mine in cyberspace. So, some repeat here..)

Nikon does a fine job of spot metering. In fact, the spot is much smaller than offered by other brands and lends to an accurate spot read.  It is easy to move from spot to matrix metering  (sort of an averaged read of the frame) as you wish.    As far as I can tell, the Nikon spot metering is the most accurate when judged by the size of the spot read, allowing a dedicated read of an area uncluttered with brights and lows.  

 I don't think Richard said that. He mentioned color matrix metering which works pretty well. That is a plus and likely why mentioned.  You certainly have other metering options including spot which works quite well (once you learn to read where to meter!).  : )

(Of course, with a zoom lens you simply zoom in on the place you intend to measure and take the reading.  A button will lock the exposure if desired.  Then recompose and shoot.   You will soon know how well you read middle gray in real life color...or how to up or down an ev to make up for it.)

Sigma make mounts for most popular DSLR cams.    I have had no reason to check on Oly and really do not know.       

I almost bought Oly but got Nikon since at the time  Oly appeared being "locked" into a system with little known by me at the time how well that system performed.  And money was (is) an issue.   And, my other cams were Nikon and a Pentax(film) with a more recent but now very outdated Minolta digital.           TomDart