I only use Wilcom, but be very careful how you use it. I trust it 100% to reduce the size of a design. It doesn't change the number of stitches, just the size of the stitches. So if you are decreasing the size the final design looks full and dense, which is nice in some patterns. If you increase the size of the design you will find that the design area doesn't fill well. A small increase is OK, but not a big one. From what I can see, Era does almost the same thing except that is limited to 40,000 stitches, you need a permanent internet connection and in a positive side, it has tools for basic digitizing, which I haven't researched on my version of Wilcom
They both convert formats (sometimes complete sections are omitted when doing this), and both will resize the design. In general, I don't think you need both. One will do. They are both free but you have to deal with the ads. Little price to pay for what you get.