Sadly, as I have discovered to my cost, there is no guarantee that any female pig, whether a gilt or a proven sow, will get pregnant the first time, or again. My last one, a sow with a previously normal first litter when she had been a gilt, appeared to be having completely normal 3 weekly heats, but when she eventually went for slaughter, they discovered cystic ovaries and all manner of problems blocking the passageway up to the womb. It is not that common, but there are definitely quite a few rare breed pigs around who have infertility problems, dozens of posts here on TAS re just that, and usually no obvious reason at all as to why it should happen, or that there is anything wrong while you are trying to get it to happen. Would be impossible to guarantee that any pig will have 7 litters - thousands probably do, but it is only after the event that you can be quite sure that is going to be the case with yours. So, personally, I think you are likely to have the same chances of getting a fertile pig pregnant whatever you go for.