Here is a review on the 28-300
http://bobatkins.com/photography/rev...00_review.html
Unfortunately very few people review sigma lens to such a technical level.
As Stephen mentioned a lot depends on your range of subjects.
The 28-300 is trying to be a one lens solution and if you need that range and don't like changing lens in the field it is very versatile. But as a rule cheap X10 zoom lens are best avoided as they tend to be very soft. So if your subject always fill the frame it should be OK. Expensive X10 zoom lens are better but are no match for x3 zoom or Primes lens in the same price range.
The x3 zoom 70-300mm on paper should a bit sharper and cheaper as the design is less complex than a x10 zoom lens.
Other factors to consider are built quaility, weight, Max aperture, Image stabilizer and Auto focus speeds.
Simple Lens facts of life.
With lens you do tend to get what you pay for and often needs more consideration than the camera body. In the scheme of things both of these lens are considered to be budget lens. If you always fill the frame they should be acceptable. It is when you start to use only a fraction of the frame the poor lens quaility starts to show.
Broadly speaking lens review (rating) sites group lens into 3 bands. There are a few exceptions to this but the general rule of thumb is well tried and tested.
1) Budget Lens ( Kit or starter lens)- upto £200 - £300
Pros - Can give very acceptable results when stopped down to f11 and you fill the frame. light weight.
Cons - Tend to colour fringe on the edge of the frame on high contrast scenes. Slow AF, Very soft at wide apertures, Needs good light, Build can be very flimsy.
2) Prosumer Lens ( often bought as an upgrade from Budget to over come some of the budget lens problems) between £200 - £500
Pros - Give very acceptable results when stopped down to f8. Improved sharpness allows tighter crops. a more ruggled build quaility, Faster AF and a wider Max Aperture.
Cons - A bit heavier and it's not a Pro lens.
3) Pro Lens over £450
Pro - Most will give excellent pin sharpness wide open or stopped down one stop.
Some have very fast AF. Built like a tank. Wider aperture for shallow DOF and low light shots.
Cons - Price, weight
Zoom lens - Cheap zoom lens tend to become much softer than higher priced lens at the long end of the range. There is no doubt that with modern design some of the zooms can match a prime. But overall the good primes still have the edge and are cheaper than equivalent zooms.
Personally I think too much is made of the versatilily of zooms especially if you are doing a final crop in Post processing. I tend to have a 300mm Prime attached for most of my shots. I miss only a few potential shots because the lens is too long.
You may find upgrading the camera to a higher resolution model start to showup the weak points of a lens.
If anybody can find any omissions or flaws in the above general lens guide feel free to chip in.