Either will give a quality jump
You are coming from the other brand you mentioned so either pair of Harbeth can really give you a huge jump in term of quality and musicality in sound.
I agree the SHL5 is fuller on bottom but not the more revealing top and different mid compared to C7ES3. The true differences between the two, to my ear only, is the C7ES3 (also all the old models are similar but not better to) midrange presentation is more dense or some say "tighter" compared to the SHL5 which midrange is loose or I think better term is "relax". Some comment the 7 sound is presence and some comment the 5 sound is "magical". I say both is magical, presence and the 7 sound tighter and the 5 sound more relax (by comparison). Both high is excellent but depend on what kind of music and presentation you are looking for.
The super tweeter do really let some instrument sound more obvious in the high, say drum cymbal or steel string acoustic guitar, than C7ES3 but that does not mean the C7ES3 can not reveal that sound but it may appear less "obvious" if you play them side by side. In practical usage, when just listen only 1 pair of speaker, either one is excellent. Even the C7ES3 do not go as low as the SHL5 but it is enough to play music with weight and authority. steveinaz wrote a nice impression regarding the bass of the C7ES3 you may take a look.
I have both 7 (older model thought) in room at Malaysia and 5 in hall at Singapore with same LFD brand amp and same Marantz brand cd player for both. When I enjoying either pair of them I do not miss another pair. Both is excellence and truly amazing music interpretor.
However, you should try to listen both pairs and decide.
"Bath in Music"
Harbeth owner since 2004