I believe I'm just about at the point of no return, with the battle preparations quest.

What I'm wondering is if it's generally better to finish the game before starting on the expansions, or should I play the expansions before beating the game? Anyone have any thoughts on the best order here?

Finish the main story, after that do Hos, after that Baw...

Play the expansions when you are of the appropriate levels (30-ish for HoS, 35-ish for B&W). When you play them is irrelevant, only the levels, for the sake of XP. Remember that they are completely self contained, and if you leave it until after game completion, most important NPCs will no longer be available, so BEWARE!

You mean important NPCs inside the expansions won't be available if I beat the game first? I'm level 30 now, so I could certainly start HoS, just not sure if I should before beating the game or not.

No, I mean NPCs from the main game. For example, if your main source of herbs and such was Tomira, you will no longer be able to find her. The same goes for the Pellar. There are alternatives, sure, but I was just giving a warning that certain NPCs will no longer be available, and that goes for ALL of the main NPCs.

You have to weigh whether you do the expansions and level up for the end game, or level up for the end game and (a) lose said NPCs and (b) be overpowered for the expansions. Your choice.

Cool, thanks for the advice!

There's 1 quest in HoS in which you meet an NPC who'll mention Vesimir. If you do that quest before the battle of Kaer Morhen, you can ask Vesimir about that NPC.

You are actually supposed to finish the main story before completing B&W. *(not necessarily before starting B&W, just before completing it)*
I don't want to give anything away, but at the very very end of B&W *(right before the credits)* there are unique character interactions at your villa in Toussaint. Whoever is there changes based on how you played the main story.

In the same breath though, you'll really only care about this if you're a 'Story' junkie like me. If you're a cutscene skipper, you won't give two snits about this :-)

It made more sense to me to finish the game before going on to the expansions without the threat of impeding doom hanging over the whole story.

The expansions are about Geralt while the main game is about Ciri.

It does make sense to just stick to the level requirements. That being said, the level requirements of Blood and Wine are higher than anything in the main campaign. Combine that with the plot elements (as mentioned in the 2 posts above mine), and it just makes perfect sense to wait until after the main campaign to play B&W.

Touissant has everything you'd need in terms of supplies. Just make sure you've done all other quests in the main maps because as mentioned some people will disappear after beating the main campaign.

Hearts of Stone on the other hand, has lower level requirements, and in terms of both story and quest level, it fits in sometime around the middle of Act 3 (Final Prerparations phase). You're already in Novigrad area taking care of business, Geralt just wants to pick up a "quick" contract, and the quest unfolds from there.