I travel to NYC from RIC about once every two months.
At this point, my go-to is Greyhound. What I've learned is:
1) if possible, book online (cheaper than at the station) for an express schedule. The greyhound express routes offer buses with (usually unusable) wi-fi and working outlets for every seat. For express tickets, the drivers have the passengers board in groups of ten (1-10, 11-20, etc.). I've learned that if I book at least a week in advance, I'm basically guaranteed to board the bus in the first or second group, which in turn means I can usually rig my way into siting alone or at least securing my place on the bus. This also means that you don' have to be at the terminal extra early just to get a spot in line.
2) Greyhound drops in NYC at the Port Authority bus terminal. This is basically in Times Square. Most subway lines run very close by, and some run directly to Port Authority. There are always a lot of cabs outside, as well.
3) I've learned that it doesn't matter what time you book your departure ticket for; your comfortability on the bus is going to vary randomly. Sometimes there are not many people, sometimes it's crowded, sometimes it smells, sometimes the driver gets you to NYC early. I now usually choose the 10am express departure to get into NYC around 5p (I pick this one because it's the shortest duration/no transfer stops). I used to travel after midnight because of school, and there really is no difference in how comfortable the ride is.
4) I've never felt unsafe riding any bus line. People are just trying to get home or somewhere else. There is security hand at greyhound in RIC, and Port Authority has cops in and aroun the building.
5) I've taken Megabus a few times, because you might get lucky on fares. I took it up once for $16. They drop off in a little more inconvenient spot, and they pick up for the return trip in the most inconvenient spot. (Not the same spot like greyhound does it.) the cheap price has to be weighed with the inconvenience, but at $16 it's obviously worth it.
6) Your best bet is to book early for the cheaper price. It always goes up in the week before by at least a few bucks. The greyhound fare for my round trip is usually around $65 (offseason, no holiday). Two days after Christmas, my one way to NYC was $75, and the return trip two weeks later was $50. I bought two separate one way tickets this past Christmas because it was cheaper than buying a round trip ticket by $20.
7) I do not park at greyhound. They don't have long term parking for customers, but you are free to leave your car on the side road. This was a little sketchy, because you car is just sitting on a poorly lot side road for a few days unsupervised. It turned out fine, but I now just get a ride or take a cab to the terminal.
8) People have mentioned getting to DC first. That was nice, because the bus ride is shorter, and Union Station is nicer/safer, but the price for driving an leaving your car there negates what you save on the shorter ticket route.
9) If you take Greyhound and you bought an express route ticket, arrive at least 30 minutes early so that you can get your bag ticketed if you're putting one underneath. One bag up to 50lbs for storage underneath the bus is free, but it has to be tagged for the handlers. You get free carry on.
10) I do not take the china bus/Eastern. This is a case of you get what you pay for. When I was walking through Chinatown last month, we walked by their departure in on the curb outside. It looked like a riot. People know have had average experiences with Eastern, but I won't book with them again.
I realize in your question that you are thinking about booking a ticket right now for a departure tonight. Please, disregard what I was saying about booking a week in advance. I'll leave the info up in case someone else was wondering.