It will be worth it if you are planning to at least double your amount of listings. You will need at least 90 listings ( and can go up to 250 + 100 auctions) for a basic shop to be viable compared to what you are paying for pay as you sell. You will also get a shop front.
If you go for a shop, try to use as many listings as you each month, taking in to account any rolling over which will come out of your monthly total.