I just got some batteries from Hobby King with XT30s on them. Annoying because I thought they were XT60s and didn't order any male connectors. XT30s seem quite nice except I don't have anything to plug them into.
XT30 are much better than JST RCY, because XT30s are gold plated for less corrosion and have more contact area for higher amp rating - 30Amps continuous. Officially JST RCY are rated at 3 amps...http://www.jst-mfg.com/product/pdf/eng/eRCY.pdf
XT30s weigh about 1.7 g a set (unsoldered male and female). Not sure about JST RCYs but I'm sure they are lighter if only because there is no solder. So we might be adding a gram here.
Both are polarized but either can be connected the wrong way with enough force and for the XT30s just the right (wrong) starting angle
XT30s are soldered so easier to use if you don't have a crimping tool of the correct shape and size for the JST RCYs pins and sockets.
XT30s look like XT60s but are about 1/2 the size.
XT30s are more expensive. JST RCYs are widely available available on pre-crimped on pigtails. XT30s are pretty much only available for a reasonable price from Hobby King as far as I have found so far.
XT30 like XT60s are correctly named female for the electrical socket and male for the electrical pins which is the common/best/ most useful way to name such things. JST RCYs are, for some reason, are typically named for the gender of the plastic insulator (at least for RC use) which is confusing.
I've been using JSTs for six years on my under 10 amp power systems. I have a dozen LiPos with JST connectors on them, and probably as many ESCs. The crimping tool you need is the same one you need for servo connectors/extensions. It is kind of a pain to use, because the pins and wires are so tiny -- hands-free magnification helps a lot. Sometimes I just use the connectors with pigtails to save on eyestrain. Too bad about HK switching to another connector for the small LiPos...I already have to cut off the XT60s from all the larger LiPos that I buy from them and switch to Deans Ultra. But there are other competitive places to get LiPos. Just yesterday I received an order for some small LiPos prefitted with JSTs from Value Hobby (link: http://www.valuehobby.com/ ). The closest equivalent HK LiPos were more expensive. Value Hobby also carries the JST connectors, with and without pigtails, and also at a reasonable (i.e. cheap!) price.
The XT30 plugs are rated for up to 30 or 35 amps (depending on the page you read) while the JST Red Plugs are rated to 15 amps. That's the real difference.
I'm pretty much committed to the JST plug for anything using a 12 amp ESC or smaller. I can't see why all of a sudden we would need something new for Mini designs.
I still see the need for two plugs. XT30 and XT60/Deans/Whatever, or JST and XT60/Deans/Whatever to handle the 30 amp and up needs. From what I've found so far, the XT30 is a HobbyKing design, and may be a solution looking for a problem to solve.
I'm not going to and spend the money to switch, and the cost of JSTs are cheap enough that I'll pull any XT30 I get and replace it.
I had some XT30 ends on a couple of batteries sent to me recently. I left a note on the store chat window. Jim called me and indicated that they were sent out by mistake. He also indicated that due to some new regulation ( I hope i got that term correct) , that FT was going to have to switch to the XT30's. They are not there yet, but coming soon. FT is shipping me an adapter so I can use the XT30's they sent me. It wouldn't surprise me if they came out with an article when they make the switch.
I received an ESC with XT30 connector in the Power Pack that I bought for the FT Tiny Trainer. Of course, the batteries all have JSTs. But the Power Pack that I purchased for the Mini Guinea had two ESCs with JST connectors. So, I borrowed one of the ESCs from the Mini Guinea--which we haven't built yet--and eventually lopped off the XT30 and replaced it with a solder+heatshrink JST connector. Much easier than working on the battery end of things.
It's new, they are trying to get it into general use, and it DOES have some good features like higher current capabilities. But, for now this is happening to others also. Easy fix. You could send it back.
I got some of both types recently (Late Dec 2015) The Power pack A had the XT30 on the ESC, the Power pack F had JST. The 800 3C batteries had XT30. They did send out some adapters recently if you want that. I got some additional XT30 connectors from hobby king and updated the ESC's. I also got a XT30 charge board so everything is converted over.
The XT30's are a MUCH nicer connector than JST, but I it's a pain to have to convert over.