Differences I see:
Level 3 spell versus level 4 spell.
Suggestion costs less SP (by virtue of the previous line).
Suggestion has a spell component cost (oh noes, because those are expensive >.>).
Suggestion does NOT have a somatic cost. (Does this matter in DDO? Can I cast it while held, etc?)
Charm Monster allows periodic additional saves, Suggestion does NOT.
Suggestion has a 1 second lower base cooldown than Charm Monster.
Am I missing something? Is a level 3 spell just outright better in every way (except for base save DC) to a level 4 spell?