Just my 2 cents fwiw.....
You can interchange the tops. The hard caps I've read have more room for
juice if their the LR ones but also burn juice faster than regular or HR carts. If you go with hard cap, choose the regular resistance to be on the safe side (if your batt is under 3.7v (I think) LR carts can wear out your batt faster - your 1st main concern is choosing the right resistance for your batt, though I'm not certain I have the math correct. Check the model specific section.
The hard caps have, afik, an inner O ring to prevent the juice from being sucked to fast and ending up in your mouth because the lower the filler in the cart the higher and faster the juice will move to the top. visa versally, the soft caps have more filler by spinning the carts like the 808 carts, reaches the very bottom of the cart. I'm guessing there's more filler in a regular or HR 510 cart so you wouldn't need the O-ring and can use either top. (I have a lot of hard tops I kept when on the KR808's changed to soft tops - the hard caps were considered highly sought after. I'd be happy to just give you some if they'll fit a 501 cart so you can try both.
I personally like the condom fill because drip tips won't get the juice to outer filler - just straight down the middle. The condom fill with spinning makes sure you don't have dry pockets, which will cause a burnt taste. Some people use syringes. I personally don't want even blunt needles around if taking a little more time (and also less risk of damaging the atty) will do the same thing. I also prefer the soft tops having experienced the hard caps falling off. At one point some were glued and you needed a small screw driver to work the cart top off to refill. Like total PITA! The soft tops stay on.
check out
This thread- click here for other tips and tricks to get the most out of any cart you go with. and, let me know if you want some hard tops if you go with regular resistance soft top carts (if you try the LR they are going to come with hard tops and the O-rings? I don't know for sure so let me know if you want some O-rings so you can try the difference, again, they're made for the KR808's but if they'll fit you might find it helpful.
Wish I could help you more on the 510's fit. Maybe someone else who's tried both can post about that. I've interchanged the hard tops with soft tops, both on 808 carts only and looking to switch to a 510 soon so don't wait for me to learn this especially if you need carts, like NOW.
My opinion is that the soft tops are all around easier and will never need to be glued on so possibly healthier without glue (it breaks off in pieces when prying the top off - please if you get a glued on, work with it over a sink, especially if you have children or pets in your household - there's zero info about what's in the glue or if anything leaks out of a seal after you break it off. You don't this stuff falling in all directions all over a floor since children and pets taste test everything within their reach....another thing I'd like to see full disclosure on but until then, use your better judgement.
btw, none of hard tops I have were on any carto that used glue.
HTH a little?