I have to replace my water pump, so I figured I'd go ahead and swap out the radiator too. I live in a hot climate in Florida. Stop and go traffic gets it running hot. I was just wondering which radiator I have now. I like the two core aluminum ones. I like to have a drain plug too.

I can't help you with the detail differences, but our host's catalog lists a selection of radiators.

http/www.minimania.com/catPage/5-5/5/5/0/Radiator--Fittings

Selecting the right one for your car depends on its build year and engine size and where you live/drive.  If your climate is moderate. the stock rad for your car should be fine for an stock 998cc. Physically they are all the same size. Functionally, they do vary. Rad efficiency depends on the size, number and spacing of the core tubes AND the spacing and shape of the gills. If the core tubes are too big, they won't transfer heat as efficiently; if they are too small they won't flow well, especially as they get choked with mineral deposits over time. If the gills are too far apart, they might not tansfer heat efficiently, or if too close together might not let air through properly.

You don't really want to "over-rad" your engine either - again the size depends on the engine, the water pump's delivery and how well the thermostat is working. The thermostat is intended to maintain the optimum temperature of the engine and should open/close as you drive along to regulate the engine. If you could put a honking big rad in it might just cool things too much and cause the thermostat to work overtime.

So, unless your 998 is built and tuned to the ultimate, or you live in a very hot area, or you race, the stock rad or something similar would be fine.

How can you tell the difference between two-core radiators? More specifically, the standard two-core vs the super two-core? 

The one I have now is in an '89 Rover 998. It's a side-mounted, black colored one without a drain plug. I'm not sure if it's a "performance" radiator or standard. What's the difference?

Thanks in advance!