There is a reason they are so cheap and unless you only want the bed for about 6 months to a year before the cheap poly foam starts sagging (and possibly causing back pain), I wouldn't buy either. On this forum, many of the "S" brand of mattresses (especially pillowtops) have been found to be flawed for long term use. Most of the folks here have experienced this first hand and are here looking to an alternative to those mattresses. Some chose memory foam, some latex, and some chose interspring mattresses with latex on top. My opinion is that latex is the way to go, but it's not for everyone. It is also expensive....good latex anyway. So you have some choices to make, buy cheap, inexpensive and replace the mattress every year or so (or live with the sagging mattress for years), or buy something that will last for 10 or 20 years. There is a lot of good info on this forum and some reviews that are misleading too. Be careful about reviews for mattresses where people have only glowing things to say, AND their reviews are the only based on a short ownership. Reviews under "Reviews" section of this forum where the reviewer only has 15 points (which you get for a review....meaning really only one posting) means that the poster is probably bogus and they've only come here to promote their product. You should think about ignoring those postings. I find them humorus but sadly misleading to consumers on this forum.
You should also use the search function to look for information. There are also posters who have 600-800 posts on this forum who have been around a long time and most of them provide very useful and helpful information. You can even "search" for their posts once you find someone who sounds reasonable and seems to be giving reasonable, logical advice. My advice is to take some time to read before buying by price. It might be a good investment of your time.
Good luck.