Tech Trading

I think that tech trading is an advantage to a player who's willing to do it a lot. If anything, the AI does not trade often enough, so leaving tech trading on probably makes the game easier.

On the other hand, sometimes it is nice to play a game without having to check for trades often, so I play with tech trading off sometimes. Bottom line, play however you like, I don't think that either way is really better than the other.

Tech trading was a bit too profligate in the early days of the game, but with the new mechanic of not being able to initiate a new diplomatic contact until after a "cooldown" period, it feels better balanced (IMO). I'm not sure, but I think the devs might have tinkered with the code over time, to make the races less likely to trade the more strategically valuable techs like military tech and extreme planet colonization.

I usually play with tech trading turned on, because it's a way to rush-buy a colony ship or sensor scout by generating a little cash during the colony rush. I try to sell to minors first, to slow the spread. Later in the game, it's one more tool for diplomacy or manipulation. Sometimes I'll sell or gift a few techs to a race I'm trying to use as a buffer or distraction against a more dangerous race, to buff them up. That can be fun, if you're careful to protect your more valuable research lines.

So I'd say leave it on, unless you're spotting something about it that really bothers you. I might get annoyed at tech trading if I consistently fell behind the other races in tech research, but I usually manage to keep up.

It's true that AI races trade a lot of techs early on, that's why they will often be ahead of you in the tech race early in the game.
However like you said already you can use tech trading to your advantage as well - in fact selling less crucial techs will enable you to expand much faster while not giving too much away to AI's - and it can be used to improve relations with AI's.

I nearly always play with tech trading on, all in all I like this more than disabling it.

To say nothing of getting some quick cash from the other races (especially minor races) for the less useful techs. This can be important if you are still running a deficit in your economy.

The game is definitely easier with tech trading on, however tech trading is really, really, really boring and I usually turn it off for that reason.

What really irks me about tech trading is in order to get the most out of it you have to trade each tech individually. For example you won't get nearly as much cash for trading 10 techs as a package as you would for trading each tech 1 by 1. And because there's no way of sorting techs by value or any other means except alphabetical you have to scroll down the list after each trade to find the ones you want to trade next.

Then once you've done that, you get to do it up to 16 more times for every civ and minor civ in the game. Screw that, I'd rather just turn it off even if it does make the game more challenging.

Well, you're right about trading them one by one, but in DA, you have to make the most of your diplomatic relations, because in DA, you can't talk to them at any old time. Only with the Super Diplomat ability can you talk to them every few weeks, otherwise it's longer to wait.

I notice tech trading homogenizes the gameplay, so I turn it off, in fact I can probably count on my hands the amount of times I've played with it on. I find it more fun, however for the more serious players that expand rapidly on high difficulty level (crippling and above), I can see the need to sell the tech for cash.