OKAY SO OPINIONS OUT OF THE WAY Second Life is a giant Social Program by Linden Labs. It's definitely got more of a "buy" feel to it, but remember that while you're paying for stuff, a big portion of it goes to the person you're buying the item from, another person IRL, and they can end up cashing out that money for Real Life money. Some people treat SL like a job, and spend countless hours making things to sell, and people will buy it and they even make a lot of money doing this. One guy made over a million dollars doing this, so it's not like it's people shouting into a bucket and hoping it suddenly fills with cash, no it's real people spending real hours and days and even months making things for other people to enjoy. A shop owner can spend a few hours making a shirt to sell and sell it for ... idk 200$L, which is a fraction of a dollar, while some other guy IRL would sell shirts like that for multiple dollars, so she's already making less than people IRL, so don't look down on them. That said, FURRY AVATARS, especially, are VERY difficult to make and take tons of time to model, and script, and texture, and that's why they're very expensive, usually 800$L for the cheaper ones, going up to $L2000 (approx 8 dollars) and more for the more intricate ones. (I actually have some of these). BONUS PART: Second Life has PLENTY of people who give out free stuff like avatars and clothes and whatever, you just have to spend a little time looking for them. It's pretty easy to find them if you just look, and if anyone needs help finding freebies I'd be happy to help you look. As for the social side of SL, it's pretty open and people are normally very nice, as long as you don't go into their private homes without their invitation. Going to hangout spots or gathering spots, or clubs and stuff is where you can find plenty of people who are being social and talking to each other. It can be hard to speak to strangers still, but just remember that everyone got on Second Life to be social, so talking to people isn't as bad as trying to talk to strangers in real life.
As for IMVU, it's a lot less of everything. The whole experience of IMVU is inside the pre-made chatrooms, and in very few player-made chatrooms. These chatrooms are pretty small and condensed and makes it feel like a bigger version of something like Habbo Hotel (if anyone remembers that chat-game). As for making your avatars, while it does have plenty of free stuff, and ways to make money to buy stuff, it still feels ... like I have little control over my character's appearance, it's very limited in that aspect too. As a chatroom, I've only visited a few places and talked to some people, but idk I didn't even like the people on IMVU. They felt too closed off and picky and grouchy and such. Especially the PEOPLE people, humans. They feel very judgemental in IMVU, while humans in SL generally accept furries since they've got such a huge following in SL. If you play SL, you're bound to run into furries eventually, even if you never go to furry sims.
At the end of it all, I much prefer Second Life. It's a chat program, but it feels like so much more, it's so huge in the grand view of things. A vast, almost never-ending sea of player made worlds, there'd be no way for any one person to experience everything in SL. One more thing. Second Life is really demanding because of how big it is. Every model and texture and script in the selected world you're on has to be loaded, so it REALLY grinds at your internet, your RAM, and your graphics card. Fast internet, lots of RAM and a good Graphics card are needed to experience Second Life to the fullest. You CAN lower graphics down and that makes it capable to play on lower-end machines, but you miss out on a lot of the beauty. BUT, if you can really only run it smoothly like that, it's STILL better than IMVU is.