As a helicopter parent whose time with youth soccer is ending, I thought I would share from my experience for those still in it. Your mileage may vary. First, the alphabet leagues:

ECNL, ECRL, NL, NL Pro, NPL, SCSCL, PMSL, etc. etc.

I've had kids in 6 of these leagues, from ECNL down to open. Does it matter? That depends on what your goals are. Do you want to play against the toughest competition? Have the best resume? Just have fun? Travel the most/least?

ECNL -- Most competitive league in the US, with more travel and more "National" showcases than most other leagues. One of the big advantages is clubs in this league don't have to re-qualify year after year. So a player can know what level to expect with less risk. Four clubs in SC play in this league. Competition for roster spots can be brutal. Some clubs show little loyalty to homegrown players, some show preference for homegrown players (I'll make another thread on clubs). Being on an ECNL roster does have clout with some colleges, helps get your foot in the door. Many players talk about ID events where colleges separate the kids out by league. Very little recruiting happens at the younger ages even in ECNL. Almost all college recruiting (related to the league events) occurs at showcases and is focused on seniors and some juniors for boys, and probably a little more focus on underclass ages for girls.

NL-Pro -- The quality of this league is probably equal to ECNL, but the difference is teams in this league are here because they won their way there in USYS National leagues (previously called regional leagues). This league acts almost as a "showcase" add on to the national leagues, with qualifying teams from the previous year coming together twice a year for 3 games over 3 days against top teams from other National leagues. Teams that qualify are very good, but teams in National leagues with multiple quality teams can miss out on qualifying due to one bad day or call. Relegation/promotion is great for professionals, but if the goal is for kids to play at the appropriate level, it is easy for a team of kids to miss out, which leads to more player movement between clubs as they don't want to go "down" a level in competition.

ECRL -- Created for 2nd teams at clubs in the ECNL. Slightly smaller area for a little reduced travel. Expanding showcase opportunities, but not as well attended as the ECNL. Players sometimes brought up to ENCL to fill in for injuries.

USYS National league -- Average quality of play and travel is similar to ECRL, but the bottom teams in the league are not quite as competitive. But, only 2 or 3 teams from SC will qualify for this league, so if you miss out you miss out.

SCSL, PMSL, Open -- Similar level of travel for all these state leagues, so really about finding the level to fit the player. SCSL teams can qualify for the National league described above for the next year, and some teams in this league are of that quality, but not all.

Does your player get frustrated playing with players who don't take it as serious? Would they rather be a star, or a cog in the machine?
The "premium" leagues do attract quality players. Playing with and against better quality certainly can aid in a player's development. The players on these teams also get the most care/attention at the clubs that have them. But even within a top team there is still a heirarchy and not all players are the same, and some are continually under pressure to try to stay on the team. If playing in college is a goal, the reality is that most of the clubs do very little if anything to help most of the players even on their top teams with recruitment. I'll make another post about colleges/recruitment, but a player playing in Piedmont National league or SCSCL on a team that attends showcases (Jefferson cup, Raleigh, Disney, etc) and goes to ID sessions at specific colleges can probably get as much exposure.