Ahh, those wonderful floor seats! My little one was almost 4 months when I started researching the possibility of a floor seat (kind of like a booster seat with more support, usually in the back and thighs). I was a little overwhelmed with all of the available different kinds out there, but also very excited at the prospect of calling one or two kinds my ultimate choice!
Number one, The Bumbo.
It is probably the most well known and most popular of floor seats out there; it has been around a while. I'll be honest, I never gave the Bumbo a fair chance. I know it is well loved because it serves its purpose in an effective way, but even though my son had skinny legs at that point, I didn't like the fact that the thighs space was a one-size-fits-all kind of deal. I love all the colors choices, though. You're sure to find one that will match your home decor or your child's personality, but that is quite a superficial bonus point.
Number two, the Mama's & Papa's Snug Seat.

First of all, I found out the brand Mama's & Papa's is known for making slick, expensive but super effective items while shopping for strollers at the beginning of the year. I love their overall designs for all their products, they sometimes look... futuristic. Very nice in structure and colors. Between 60 and 70 dollars without a sale for the floor seat though, you have to really think about your choice. What I suggest is to find one used, sold by moms on one of those online swap websites or groups. The snug seat is so robust, it's obviously made to last the strain of use of a few children for sure. A few generation, even. I used this one when my boy was 4 months, and he loved it. I think it gave him just the extra support he needed, since he could certainly hold his head up on his own already. I used it for a few minutes at a time for him to play with his toys, but I also began using it for starting solids (well, purees...) and it was essential! It's very convenient for travelling and bringing from house to house, as well.

Thirdly, the Inguenuity Baby Base (2 in 1)

This one was brought to my attention while I was looking (and had already found) the Mama's & Papa's one. I couldn't give it a chance right away, even though the design intrigued me more. It was only on my birthday that I received a message from my favorite baby boutique telling me it was on sale (I'm subscribed to those kind of notifications), from 50 something dollars, to 30 something, new! I thought it was faith, and even though I wasn't around to buy it on that day (we were on vacation), I had someone go get it for me. I wasn't going to pass on that chance again. Needless to say, I was super excited to come home and take it out of the box to examine it and let my son have a try. I just love how the table tray clicks into place and can be hidden underneath the seat when not in use. Also, it is light enough to carry around (though I wish it had a handle or something I could easily grab onto while lifting it up). At the same time, it is strong enough to keep a baby well grounded. It is possible that it doesn't give as much support as another seat, I couldn't judge that element well since my son is now also 5 months old and carries his head and back very well. A plus for this one? I'm a big fan of the bright aqua or pink, but I do wish there were one more color choice, perhaps. A more neutral (but just as bright!) one. How about orange or yellow? [Update: My prayers were
readily answered! Hopefully, those other colors will become more widely available soon.]

Lastly, the Summer Infant 3 Stage Super Seat.

The look of it alone was very exciting to me, and I certainly focused on how multi-functional it is. It is kind of a cross between a booster seat and an exersaucer, and that can be fantastic for some moms, since you can save on a couple of purchases there. I wasn't able to find one second hand at the time of my search, but they're out there, and seem well worth a try. You can either keep your baby entertained in there, while being well supported, or use it at the table for meal time. Then again, my son personally, has plenty of loose toys already, without having to add those attached to his sitting devices. In the end, I wish I could have tried it, but I'm content with my final choice as well.
For now, I paid forward the Snug Seat to another needing mother, and stuck with the Baby Base. I think my son will appreciate as well, and who knows, his future younger siblings might too!
What kind do/did YOU use?