I don't think most people who are looking at secured credit cards are expecting to apply for a mortgage in the next few months. Or at least, not usually hoping for prime rates. This is the rebuilding credit forum and secured cards are a tool used to rebuild credit. I dont have any long-term expertise, but from reading what people have said here and elsewhere, secured cards are calculated the same way in credit scores as non-secured cards. They may be less impressive to loan officers, but, that's a bridge to cross later.
As for whether it is better to use the money to pay down a debt instead, situations vary. For example, in my case, I have no debts to pay down. I just need credit lines. So, that is an easy choice for me to make. I have no idea the OP's specific situation, but there are cases where even someone with debt may be better off getting the card, so it can start aging.
To answer the OP: I applied for Discover It and got rejected when my scores were 625/520/520, and tried again (rejected again) when my scores were 635/580/580. My scores are now 667/656/652. I was preapproved today, but too chicken to click the button to apply for real. I have no idea why a secured card should be kind of hard to qualify for. In the meantime, I got a Primor card, which has a much worse reputation for customer service, but because it is my third card, I am not sure if I'd really benefit from Discover as a fourth card now. I could cancel the Primor if I got the Discover, but, key word "if."
Due to this thread, I broke down and tried again, and this time, I was approved for the Discover secured card! Planning to close the Primor card (haven't received it yet and it hasn't even appeared on my credit reports yet, so I'm not losing credit age or hurting utilization percentage).