I’ve personally purchased from both stores, and was sponsored by Sheinside for a little while. The quality of the products are hit or miss, I’ve got a few reviews for Sheinside on my blogspot that you can read, if you’d like.
I have always received my items on time, without having to contact customer service. My stuff has always been in good condition, without damage or screw ups on my orders.
That being said, I can’t vouch for everyone. I have seen some negative reviews, and both stores have been known to steal artwork from well-known artists without giving them any credit. If you plan to purchase from either store, I’d definitely recommend making sure that the designs on the clothing haven’t been stolen. I’ve found that a lot of the “all-over print” sweaters on both sites are printed with art they took and used without the artist’s permission. And that’s pretty shitty of them.
Personally, I have had positive experiences, but some of their products (on both websites) are ridiculously overpriced, and some are of poor quality. You can find many of the same products on Taobao AND eBay for half the price they sell them for. However, on the items that I couldn’t find on eBay/Taobao that I purchased, I’ve been fully satisfied with my order. (I usually buy when they have sales, though, to make sure I’m not losing too much cash if I’m disappointed with my order.) Just be smart, continue to do research, and make a decision that you feel most comfortable with.