I feel that a correctly preformed sigil ritual can be more effective than a poorly preformed evocation, and there are levels of success to be had with both methods.
Sigil ritual generally requires less to succeed in terms of both psychic senses and physical investment as far as tools go. Sigil ritual can be done informally and on the fly, and is generally considered an appropriate introduction to a spirit. whereas a proper evocation will require a alot more planning, psychic development, and a general relationship with the force summoned to say the least.
As far as the actual "power’ diffrence goes that may just be a petty distinction that we make to ourselves its a mystery to me personally so I cant say for sure I do believe that a proficient person can do more with a sigil than a resource rich beginner could do with all of the ceremonial tools in the world.