Not a definitive list but I think of '65 as being lighter and easier game system, not that either game is really in depth or "over-complicated". Both games are basically alternating impulse activation games. When a unit is "done" it is done and somewhat helpless. So your tactics have to take that in to account
In '65 your units are activated by card play - Everybody activates each turn but some of what you can do is limited by which cards you hold at the moment. Each units is basically activated once, but cards and situations modify that heavily.
In 'Nam you decide which unit to activate and there are group activations, but what a unit can do is somewhat limited by what your troops can "see" and spotting other units is the key. One activation a unit can take is try to spot enemy units.
I find the LnL games much easier to play solo because of the spotting mechanism and a little more detailed in play. The mechanics are very different for each system, but the strategies are the same (thanks to the same tactical focus and impulse activation). One thing that LnL games have a reputation for is scenarios that are "solvable" and though I certainly have not played them all or played them a lot (lack of local opponent issue) I have "solved" a couple of them and though it has been long enough since I played them it happened frequently enough that it got annoying. I have not played '65 enough to see that as an issue (same lack of opponent issue).
The are different enough that I am quite happy to own both games and series. It helps greatly that I enjoy this scale and type of game and I REALLY enjoy impulse action gaming. I mostly play ATS these days.
Both '65 and LnL Tactical are Mark Walker Designs.
I don't know why but every time I play LnLT, it results in an intense excitement level. It's a magic I can't explain. The only gripe I have about LnLT is that it requires three dice rolls to resolve a combat. One 2d6 to get to a modified Attack factor; another 2d6 to get a defenders modified value, compare the differences, and a third 2d6 against the defenders morale to resolve damage.
'65 has NO dice rolls. Just subtract the defenders terrain cover factor from attack factor = number of cards to flip. Look for a "HIT" on each card. First HIT 'shakes' the defender, second HIT flips it to its reduced side, third HIT kills it. '65 also has mounted boards.