I titled this admitting that I'm a newbie so hopefully more veteran players can add/correct if I'm wrong.
I have spent strange coins on Xur items 3 different times now. I feel like I should have done more research or something the first 2 times.
Hopefully this helps other newbies avoid some Xur mistakes.
Xur sells 280 exotics and 2 engrams.
One says legacy. - that's only good for year one exotics
The 2nd says exotic and it's GUARANTEED to decrypt at 290/310
This is important because if you look around at other posts, no one gets 310 decrypted (maybe?) unless you are over a specific light level (most say 301+)
But regardless this is a really quick and painless way to up your light to around 290 and become "eligible" for harder raids.
But the downside is that you know what perks are on items that are sold at 280 and you can just infuse?
Well, as a newbie, that's certainly true but (again as a newbie) the exotic shard needed isn't a "cheap" price tag.
If I had to make a suggestion, if you are below 280 light, spend legendary marks on vendor items and infuse those as you get better items (wait till the difference in light is 6+ to infuse. The math works better this way). The nice thing about vendor legendaries is that it's "cheap" to infuse and still have nice stats.
As for Xur, save and spend for the armor engrams. You get some potentially nice class specific gear with decent perks. For weapons use vendor rep weapons (legendaries) if you really want since those again are cheaper/easier to infuse/level.
Hope that helps some other new player :)