To be honest, I really feel that either work. A lot of people will say that the long tables are not as good for conversation, but I can also see how the rounds may not be good for conversation either. So I would base it off of what you want your aesthetic to be in your space. It is very popular now to do a mix of shapes as well to give more movement to the room.
If you have under 50 guests at your event, you are more than welcome to skip the escort cards and just let your guests seat themselves. The only caveat I have for this, is to make sure you have a table, or tables, reserved for the bridal party and family. If you skip this part, the bride and groom may end up at a table that they do not want to sit at!
If you have over 50 guests, I highly recommend escort cards. An escort card is what guests pick up and “escort” them to their table. This could be anything from a tented card with their name and table number to a tag on their favor which they take to the table with them or a map on an easel with names in alpha order. One big thing that brides tend to forget is that the escort cards need to be in alphabetical order, not table number order, and it is so nice to hand them over to your planner or catering manager in that order. Saves tons of time!
If you are having more of an upscale event, or if you want to assign specific seats at the table, you would start with escort cards to get the guest to the table and then you would have place cards in front of each seat showing where that guest would sit. This is not necessary, but some couples find that they prefer this so that they know that people sitting next to each other at the table will make conversation.
One thing to consider when building your seating plan is that your guests will really only be seated for 1-1.5 hours. Most of them are all just waiting for the dance floor to open! So don’t stress about it too much!