The cricut and the sizzix (including Big Shot, Cuttlebug, etc.) are two very different animals. The cricut is an electronic machine that works with cartridges. Each cartridge includes several dozen and up to 700 different images, each of which can be resized. The others are manual die-cutting and embossing machines. They use fixed dies or folders that cannot be resized. Some of those dies are no less expensive than the cricut cartridges.
I have both the cricut expression and the Cuttlebug. The cricut takes up more room and requires a mat. It does not do embossing. Many of the carts can be purchased on ebay for around $20 each. The Cuttlebug is a lot smaller and handier. Sizzix alphabet sets (that also work with the Cuttlebug) are more expensive than the cricut font cartidges, take up a lot more space, and are not resizeable. However, they are very handy to use with a small manual machine.
Each type of machine has its pluses and minuses. You may want one of each -- the cricut (which you already have) and one of the manual die-cutting/embossing machines. I suggest you research each of the manual machines to see which one will best suit your purposes.