I find posting on Poshmark to be easier for a few reasons:
The photos are automatically cropped into a 1:1 ratio and you can adjust them while upload a new product.
You also don't have to enter a product's brand if you don't want to on Poshmark however, Mercari requires you to enter something or type "no brand". It's a small detail but when uploading many products, it gets pretty annoying.
You have to either know or guess how much your item will weigh with packing on Mercari. This has caused me a few problems in the past because I guessed the weight off by a few ounces and had to pay extra shipping at the post office. With Poshmark, it is a standard shipping cost so you don't need to do anything extra.
Round 1 Winner: Poshmark
Getting Products Attention:
Poshmark is very time consuming when it comes to getting attention on your products. You usually need to share to followers, share to parties, follow other people, share other's products, etc.
With Mercari, you don't really need to do a lot (compared to Poshmark). You can "promote" 10 products a day which basically means they will increase the rank of that item for that day which means more people will see it.
Round 2 Winner: Mercari
On Poshmark, most people comment under your products which means that the messages are public. There is a lot of spam messages on Poshmark so customer messages are sometimes lost between them. Most people's comments are generally kind, but I get some rude/hateful comments a few times a week. There is a "spam" option within the "flag" option so you can remove hateful comments.
On Mercari, messages are private between buyer and seller. People can't leave public comments on items. I also haven't had any spam messages yet.
Round 3 Winner: Mercari
Mercari takes 10% of your listing's price.
Poshmark takes $2.95 for items that sell for under $15 and 20% of items over $15.
Round 3 Winner: Mercari
Even though Mercari 'won' 3-1 , Poshmark is my current favorite selling platform. It is more difficult and time consuming to use but sales are very slow on Mercari and you can't do much to change it. With Poshmark, the more hours I put into it, the more sales I make.
In the end, I prefer having them both. I can upload products on Mercari and leave it until something sells or I get a message. And with Poshmark, I can make sure I get sales by putting in lots of hours.
Hope this helps,