I'm just curious here, is this light a really popular buy?
I mean B&H Photo Video lists the Arri 650w fresnel as being only $9 more than the Arri 300w fresnel.

I was thinking about buying the Arri 300w fresnel to use as a kicker, back light, or eye light, or whatever.... but for the extra $9 wouldn't I be safer to get the 650w fresnel and just buy a 300w bulb to put inside? That way I have more flexibility of using up to 650w if I need to?

I already have an additional Arri 650w fresnel so I'd probably be even better off since I could share the accessories (barn doors, scrims)

Am I overlooking any advantages of having a smaller Arri 300w fresnel?

Well, the 300w is certainly smaller and lighter.

I'm not super-impressed with the balance of using 650w key and fill with a 300w back. Specifically with the Arri fresnels, one place I frequently work has a kit with 2 650 and 2 300, many times I've wished it were 3 650 and 1 300.

The 300 can function as a back but I find myself wishing for more. It makes a pretty good sider and it really is a fine little fresnel. It may be a bit much for an eye light.

If you're always using chimeras or bounce cards on the 650s, the 300 is a closer match.

In summary, yeah, I'd definitely go for the 650. YMMV.

The physical size and weight come in handy in tight spaces, and if you have to boom out the backlight. If those are your needs, thats your light.