I feel it is impolite to hijack someone else’s Show HN to promote your own, like-for-like product, that too as a top level comment. Not sure about HN norms around this though.
Besides, I always bookmark threads for apps that look interesting and check back later when everyone else has posted their similar apps and been commented on, then I can compare and choose whichever one looks best (or whichever is open source).
Personally, if I find that my app is similar to an ongoing Show HN, I like to wait for an opening where I can respond with a plug for my own app.
I don't feel comfortable talking up my own app unprompted. Feels like making a big attention grabbing announcement at someone else's wedding.