If you do a quick search on the forum you'll find the never ending debate about the merits of both, and technical exercises on the whole. I think I'm in the same position as you- being 'ungraded' but playing pieces around grade 6 or 7. I did Hanon 1-10, and Czerny op 299 no 1 and 3. I had issues with Hanon and my hands developing tension and it becoming painful, to which my teacher back then told just practice slowly, but to be honest I didn't like Hanon that much.
Czerny seems a lot better and I think somewhere on this forum Bernhard or someone else mentioned that some of the his etudes were devised in teaching Liszt and were based on technical difficulties within Beethoven sonatas. Upon listening to a lot more of Op 299 I can see the relation. They're a step in the right musical direction compared to Hanon, but still the etudes I listened to in 299 didn't seem like fully developed musical ideas. Anyway, the two etudes I did seemed to help with my weakness in playing scales and sequences of broken chords. I'm currently looking at a few other etudes in Op 299 to learn.
I'm kinda hi-jacking your thread but these are questions I've been dying to ask lol (currently I'm teacherless...sigh...). What's up with the metronome speeds given in Op 299?! They seem impossible! The fastest I've heard them play is about quarter note=160, but they're notated at half note=108! Also, is there any technical merit to playing the etudes slowly and working your way up to a decent speed, or are some of the etudes technically in the same class as Chopin's? ie grade 8 and beyond, thus not making much sense for someone at my level to attempt?