most spells used through a focus only work while the wizard is focused on them ( kind of why they are called focuses ) most wizardly spells that are done on the fly or quickly only last as long as their casters power and focus allow. For long lasting things that requires more ritual and time. That is why I feel like this would make more sense. Having both of those spells on one small item would be quite the feat but they way I built Ragen I feel like it could be done. Not easy and she wouldn't have a lot of those sorts of things just laying around, but she could do it. 

Small items have advantages and disadvantages. They are less cumbersome, clumsy, and very easily concealed. They are often depicted as quicker to fire up as well, the force rings just go BOOM with a thought where as his shield bracelet takes more time/effort and any workings with his staff takes even more time and effort.

But, on the other hand they are easier to over load ( Like, if you tried using this ring to hold a dragon in place it probably wouldn't be able to handle that kind of strain. Like wise if you had to hold a shield to long against an extra powerful force.)  they also tend to be less flexible tools. ( dresden can do all kinds of shit with his staff but his force rings only shoot force, the blasting rod only does blasts, ect )

god I could go on all day about my interpretations of the mechanics of the Dresden world xD