If you go to Carls Sail Calculator (Sail Calculator Pro v3.53 - 2500+ boats
) and enter both boats for comparison, you will find that the Morgan 34 is better than the Morgan 35 in several ways:
The capsize ratio of the 34 is better, and the comfort ratio is much better, than the 35. Most of what this is a result of is the greater weight in the ballast of the 34.
The theoretical hull speed of the 35 is a bit higher than the 34, but bear in mind that the 34 can do better than 7knots quite easily; I would not rely on the hull speed numbers in this case.
The popularity of the 34 is reflected in the greater numbers that were built.
In my opinion the 34 is prettier too.
A factor in buying a 34 is the interior configuration. Unfortunately many of the 34's were built with the galley starboard; which makes the salon awkward and unsafe underway. They were done this way to pick up an extra berth, so that this configuration could sleep 6 instead of 5; the galley starboard has 2 berths underneath the cockpit.
I hope this helps.