So - here is my 2 cents worth. Both are more than likely excellent machines imho. I have a Pfaff Creative Sensation (which I think is in the same machine line as the 4.5) and a Creative 2.0 as my back up machine. I love my Sensation and sew on it every day as it is the machine I use to piece on, The Creative 2 is a great machine as well. The Pfaff IDT is what sold me on Pfaffs years ago. IDT is their built in version of a walking foot / even feed. I would not sew without it now and the Sensation is a workhorse. At the beginning of the pandemic I began making quilts and table runners to sell at craft fairs. For three years now I have sewn every day and can not begin to even think of the number of stitches it has done with no issues, The embroidery unit is excellent as well (imho). I can not answer any questions on free motion quilting if that is something that you want to do as I have a longarm for that.
I expect that the Topaz is an excellent machine as well. I used to work for Viking and their machines (at least the mid level and up) were always good quality machines but I have been out of that loop for several years. I just looked at the Topaz features which it appears that it does not have an even feed capability. The question for you to answer is that something that you want?
Bottom line - sew on both. I bought my Sensation used as a private sale. I told that gal I wanted to sew on it and see the embroidery unit work. I brought thin and thick fabric as well as some small pieces of stabilizer to try out some of the built in decorative stitches, I would ask when it was serviced and where.
Last bit of advice from someone who sold Vikings at a dealer - if either machine is being sold by a dealer negotiate !! They may be firm on price but classes, single hole stitch plates, feet, warranty are all negotiable and they might be willing to throw something in to finish the sale.
Good luck with your purchase!