Illustrator is completely different from Photoshop! It uses vector whereas Photoshop uses raster. Illustrator defaults to CMYK as the color mode, Photoshop defaults to RGB. Illustrator is mainly used for print and Photoshop is used for web and photo editing. Theres plenty of other differences that I could get into, but those are the biggest differences. It would be impossible to get the same effect using Illustrator rather than Photoshop and vice-versa.
I dont mess much with photography and such so Topaz prolly wouldn't be of any use to me...I do a little bit of photo editing but its not my forte I guess you could say. I'm a print designer, so Photoshop is helpful but Illustrator and InDesign are both very necessary.