I have done both - particularly hiring a guide to take me to Alexandria after all I have heard of it...and I had visions of its ancient library. Now, the new library didn't disappoint, but I could see how some might be disappointed with it if done at the end of the lower egypt visit. (I did it first full day in country so may have been more open to what it had to offer).
One needs to remember that Alexandria was turn of the millennium - not as ancient as south, so the influence is Roman. There are a few Roman ruins, and as an ocean port it is rather pretty, but don't go expecting 'wow' moments. There are certainly better Roman ruins around.
I am not sure I have anything else to suggest in its place, except for either a revisit of something you haven't seen enough of (I went back to the Cairo Museum as it was too fast a pace with the tour, and went to the Pyramids at my own pace because I knew it would be too short on tour). There are other, older pyramids that might be of interest....or do a cooking class or just chill.