Hello,
Some time ago I saw a YouTube video made by an experienced daytrader. I have the link but I don't remember which one it is. The daytrader said that it's best to use the Per-Share commission plans over the Flat-Fee plans because of quality of execution. Is this true? Do the Per-Share plans get better executions, share prices, than the flat-fee commission plans?

For a 1,000 share trade, if the per-share plan gained even a penny advantage over the $5.99/trade flat-fee plan, then the per-share plan would be better:
Per-share cost: $5.00 + $0.006*500 = $8.00
Flat-fee cost: $5.99

If per-share plan gains a penny over flat-fee plan for 1,000 shares:
1,000*0.01=$10.00
The per-share plan wins by $10.00-8.00+5.99=$7.99

Does anyone have any experience with both the Per-Share and Flat-Fee Commission Plans?

It is my understand that the commission plan has nothing to do with the execution. The way it was explained to me is if your trades average between 800-900 shares or less, then it may be better to go with per share, above that it was more advantageous to go with flat fee. Assuming you trade. flat fee goes off number of trades, not shares.