I only have experience with one, but what a great tool, I bought Ryobi 729 iin 2000 . Bump head uses .090" string. This tool is one of the best all time tools I have had. It has cut acres of tall wet grass and ploughed tons of earth while edging/ weeding-yes abused to weed down to roots of unwanted plants. The two bumpheads I use are so worn you can hardly read the instructions on the plastic.
I took it apart 2 years ago and lubed the pinion gear. It has never failed to start. Use Poulan two stroke synthetic oil. Keep the little plastic foam air cleaner washed and oiled. Use a platinum plug.
Over the years I have read a lot of negatives about the Ryobis. Don't know if I got a ringer or not, but if it failed tomorrow I would keep on praising it. I do not give out accolades as a rule.