Bacchus,
Never be afraid to admit you know "nothing" about anything. Lots do know everything about everything & are found out in short enough order. That you'd state it flat out adds to your credibility in my book. Nuttin' the matter with being a newby - we all were at some point in time.
If a hearty welcome has yet to be extended - consider yourself "part o' the gang" here at TFL.
I'd suggest a few days worth of TFL's excellent search engine - located at the upper right hand corner of this page. Type in ".308," ".243," whatever cartridge you're interested in and read the various topics. Probably take you a week to read all the replies & you will certainly get some good (albeit biased
) information from those who either love, or detest any given cartridge. A big +, is they'll usually state why & that's all part of the learning process. It adds up.
After all that, & perhaps during, ask some more. I'm pretty certain that is just what TFL is here for.
As to .243 vs .30-30, the first is a smaller diameter bullet (.244") based on the .308 Win cartridge. Shoots bullets (generally) in the 75-100 grain weight range & is very suitable for small varmits to deer in the 150 lb range (lots of variables & has been known to take elk/moose). A fairly falt-shooting cartridge, which means the variables in its trajectory are less than some others (the .30-30 being only one). A usually very/mostly inherently accurate cartridge due to its design - & rifles it is chambered in.
.30-30's claim to fame, I'd say, is more in the type rifles it's used in - handly, light & quick pointing lever action mainly, although there are certainly others. It shoots .308" diameter bullets mainly in the 150 - 170 grain weights & is an excellent short-to-medium range (through 150 yards or so) cartridge for deer-sized critters. It too has taken moose/elk, bigger stuff. In lever actions, it is "restricted" to flat-nosed bullets due to the lever gun's (usual) tube magazine. Pointed bullets lined up nose to tail, can, through recoil, cause the cartridges in the tube to fire - a generally bad thing. This "restriction" as to bullet shape causes some "difficulties" due to a less-than-desired bullet profile for optimum trajectory - a pointed bullet flies "further/better" - sorta.
This just a quick dump. I'm a .243 snob - freely admitted - & that of another couple cartridges. These "others" seem to me anyway, do better what a .30-30 just can't do as well. .30-30 does have its place & its usual platform is a very handy rifle indeed. Of course, so are the bolt guns usually associated with the .243 - different strokes ....
What you plan to use the rifle for will make all the difference what type you "should" buy. A .30-30 for 300 yard prairie dogs just seems silly to me & while a .243 will certainly do Alabama whitetails at 80 yds, a .30-30 may be more "right" for you in that application.
At this juncture, & if I'm reading you right, I'd read up, ask a bunch of questions, get the chance to shoot a few & make up your own mind.
I think you'll find, for the most part, the TFL gang more than willing to answer any of your (not) "stupid" questions.