My suggestion is to get cartomizers rather than cartridges. They perform better and yes...
"They perform better" is highly subjective. Performance of a cartomizer depends on the manufacturer, the resistance, coil design, capacity, and the juice used. No single combination is the best for everyone. I use whatever seems to work best with each juice I try. have some atties I always turn to for tobacco vapes as well as various cartomizers that are great on the go. And just to clarify: cart is just the little (often pre-filled) mouthpiece placed on top of (or slipped over) an atomizer and cartomizer [carto] is the "all in one unit" that doesn't require a separate atomizer. Even if you end up preferring cartos 99% of the time, having at least one attomizer around is nice for taste testing. Atties can also be used without carts (replace with a drip tip or just remove the filler from one cart) by just dripping a couple drops into the atomizer.
what would be a better kind of juice, with vg or pg?
PG/VG - you can pretty much any ratio of PG/VG juice from 100% PG to 100% VG. Again, everyone has a preference and a very common ratio is 80/20 or 70/30 PG to VG. Some are sensitive to PG although that is rare. PG is thinner and high-PG ratio juices are therefore generally easier work with in carto's (>~50%VG is so thick in some cases you just can't use it in a carto). Still - it varies by juice. I've had a 70/30 VG blend that was not usable in one carto, but was fine in another type. I've also had a PG juice that was so thin, I couldn't use it in some atomizers because it was just too damn leaky. VG tends to produce thicker vapor and a more "full" sensation in the lungs, but PG tends to let more flavor and throat hit through. These are just generalizations though - there are throat-hit boosting additives as well as a few VG thinners.
As I'm sure you are catching on to, there's lots to experiment with, and the best advice I can give is to do exactly that. Yes, getting sound advice is imperative at the outset, but at the end of the day vaping is a personal experience. What you like may end up being very different from what someone else tells you is the perfect vape.
Any simple mod with a 510 connector, or an eGo (or similar e.g. e-Power or Riva) are great alternatives to start with. I wouldn't get too hung up on voltages per-se (e.g. 3.6/3.7v batteries usually charge to 4.2v and I believe the eGo is regulated to 3.4v with an actual battery voltage that is higher). With a 510 connector, you can select from a rather large assortment of atties (including one of my favorites, the 306) and cartos. Given the larger capacity batteries, you also open up a greater range of usable resistances (lower resistance = higher current = hotter/stronger vape given the same voltage). Nearly all mods and eGo's do well with anything >=2.0ohm (I'd look around on that number for a specific device you select - some people use the 1.5ohm dual-coils fine, and for others something that low just refuses to run on an eGo). The 510 connector can also be attached via an adapter (in most cases - some devices have physical limitations) to connect to even more atomizer styles for a couple dollars. A Riva can pretty easily be found for under $40+S/H ("Lite" Riva 510 @ LibertyFlights is just $30).
Best of luck and enjoy!