I dunno David. Both systems need playtest, and both have issues that need to be resolved.
Weapon Use Task
They both use the task system for operating weapons.
The difference is in how the combat stance changes the chances of inflicting damage.
T5.00 increased difficulty with damage - although it did not appear to be one-for-one. There appear to be restrictions to the combat stances which pits one alternate stance over the other. This in turn apparently affected the tactical decisions your character made. That kind of nuance is probably a good thing;it allows nuanced choices without being fiddly.
T5.09 increased the number of attack rolls made, taking a more simulationist route. If the stance restrictions are the same, then the results are likely similar as well.
In the end, though, T5.09 appears to deal more damage overall per attack, due to the likelihood of damage multipliers, while T5.00 might deal MORE damage per HIT, due to adding dice to the damage rolls before multiplication.
It is this last point that makes an impression on me.
Consider: we are already rolling multiple damage rolls for some weapons. Adding one to two dice, PRESUMABLY TO EACH DAMAGE ROLL, might well make combat fast enough. If you can hit them, you can also damage them.
Now consider T5.09. Now you are also making one to three attack rolls. If they hit, you make some number of damage rolls per hit. That's fine the first time, but not the third time, and combat should be able to last four rounds and be a decision making process as much as a dice-rolling one.
Note that simulationist combat might ONLY last a couple rounds anyway. But, deadly as it is, I don't necessarily want Traveller combat to always be that deadly.