They're both plenty good enough for professional work, meaning that if you use Corel and Grammatik as learning tools, you'll be learning on a system you can use when you hit pay dirt. It's also not very hard to convert from one system to the other. Best of all, the Corel suite is dirt cheap. Disk space is cheap. Learn to use it all, which means installing it all.
Disk space is cheap.
Incidentally, Corel Office Professional 7 works just fine off a parallel-port Zip drive. You can keep your million words on a Zip cartridge, and chances are they'll still be readable years from now. Most writers manage to rewrite and sell their early material. In the trade, it's known as selling your trunk. Put them in a room with no telephone, be sure there's neither modem nor games on the machine, and spend several hours a day seated in front of the screen.