Honestly, if I had to buy new, I'd probably get receptacles for the same reason that new construction is done that way - it's cheaper. If I had either one on hand, that's what I'd use, but for 12 bucks / receptacle, and 30 bucks/breaker, I doubt I'd end up with a breaker on hand. That's just me.
So, yes, GFCI on the first outlet, the rest wired from the 'load' side of it.
As for the 15/20 rating on GFCIs, that can be confusing. Let me see if I can floc the water a bit. I mean, clarify...
The circuit capacity is determined primarily by the size of the wire. Max on #14 is going to be 15 amps, #12 max'es at 20*. The breaker is then sized appropriately for that circuit. Downstream devices are also to be sized appropriately.
On a 15 amp circuit, only 15 amp devices should be installed. These are receptacles with parallel blade slots (NEMA 5-15R). On a 20 amp circuit, receptacles with a 'T' slot (NEMA 5-20R) may (would) be used. This allows for 15 or 20 amp appliances on 20 amp circuits, but only 15 amp appliances on 15 amp circuits. So far so good.
Now, we want to consider the GFCI receptacle. There are two distinct aspects of these units: 1) the receptacle, and 2) the GF protection.
1) The Receptacle: They are available as 15 amp receptacles and as 20 amp receptacles, depending upon the rating of the circuit into which it is installed, just like any receptacle. It is, after all, a receptacle, and must function the same and adhere to the same standards as a normal one. NEMA 5-15 and NEMA 5-20, respectively.
2) Protection: As a general rule, the protection circuit of the device is not dependent upon the rating of the circuit into which it is installed. I want to say that again... the protection circuit of the device is not dependent upon the rating of the circuit into which it is installed. This is because they are designed to trip when they detect a current loss of roughly 5 milliamps. That is it's sole protective function. It does not trip on overload, nor on short circuits - that is the breaker's job. Yeah, let me repeat that, too. It does not trip on overload, nor on short circuits. GFCI devices do not provide overload protection, although if it is built into a circuit breaker, the 'breaker' portion does handle that aspect.
It merely compares the current in the hot and the current in the neutral, and if they differ by 5mA, it trips. It does not even need a ground connection to do this, although you always want a continuous ground conductor. Now, having said all that, there is naturally a limit as to what currents the detection components can handle, and that is where the second number of a GFCI's rating comes into play. These are typically built to be able to handle a 20 amp circuit (meaning that it can safely and dependably interrupt a 20 amp circuit), although you may find other values.
So that boils down to either a 15 or 20 amp receptacle, combined with a protection circuit which can handle a 20 amp circuit capacity. A 15/20 rating on a GFCI receptacle indicates a 15 amp receptacle (NEMA 5-15R) and a capacity to provide GF protection for a 20 amp circuit. It does not necessarily mean that you can install 20 amp outlets downstream - remember, that limit is set by the wire size and breaker.
That will conclude today's session on GFCI outlets. Thank you.
*Other factors may limit the capacity of these wire sizes. These are MAX limits.