What provider would you work with? I've heard of Enthusem and Pebblepost.
It's a well designed card with their logo, name or website on it.
Your response rate depends on a couple of things
1 - what you're selling and whether the recipient has heard of you or has the need right now for your product/service.
2 - If you are already known, you can get response rates over 10%, particularly if you are trusted.
3 - One way to improve your "response rate" is to do a follow up by telephone to check they got the message and to elicit a reply verbally.
4 - You will get the best results by working with someone experienced in Direct Response Mailings. This is a skilled position - do not expect high % returns without expertise in creating the mailing asset. If you're inexperienced buying direct mail services, I suggest meeting a few agencies for a "Chemistry" meeting where they will show you their work and ask you about your business needs. This will educate you about the process and likely outcomes.
Lastly, both the services you suggest seem good, I've not used them. But a competent Direct Marketing Agency (like mine!) will do a similar job of customized direct mail pieces as these businesses. Which may be much cheaper. It depends on how big your database is as to which is a good / cheap option.
My experience has been 5-10%. I've tried direct mail for several different ventures over the years and have not ever been all that impressed with the results.
I'm a previous founder of 3 SEO companies but I'm not biased in saying that inbound lead generation via the web is almost impossible to beat as far as customer acquisition costs are concerned compared to traditional marketing like direct mail.
Those stats are fairly well established. Of course, there may be a particular demographic you are targeting that have tested positive for direct mail affinity and in that case you will see better than average results.