I don't know if you've found the information you were looking for, or if you will be back to read this post. I can't answer your question of comparing the stiffness of the two knives you list. I can however provide a generic answer to your general question regarding fillet knives. You stated to are not interested in a deba which is a Japanese fish butcher knife. You accidentally stated a honesuki which is specific for poultry. You stated you are interested in a flexible knife and you are on a budget. The knives you linked to are boning knives and not specifically fillet knives. Generally, fillet knives are more flexible than boning knives. There are different flexes available in boning knives ranging from stiff, semi flexible and flexible but generally they are less flexible than a fillet knife. You never stated what fish you will be filleting however one of the knives you asked about has a 5" blade. Make sure you are sizing it correctly to your needs. The artifex fillet that lunatic referred is a good suggestion and if you are on a tight budget, there is nothing wrong with the victorinox for $20. http://www.chefknivestogo.com/fofikn7.html
however, this is currently out of stock. Without more info from you , a more detailed recommendation cannot really be made. These are just assumptions. I hope this helps and if it causes more confusion, post a few more questions about what you are really looking for if it's more than just wanting to compare the two knives you listed.