ATB flowstar 1500 and Laguna 2000, are basically all the same pumps. (Laguna 2000 has a little more flow) The Fluval sea is an Askoll motor, but has different volute and the pump is supposed to not make as much heat as the Laguna.
The Laguna does not come with a stand, but it comes with attachments to connect flex tubing. If u plan on hard plumbing your return, it's a little tough with the Laguna.
$120
The ATB flowstar 1500 is just a Laguna pump with a stand and a threaded union attached.
$270. (Rip off IMO. They charge you $150 for a stand and a union)
The Fluval sea is little bigger and has a threaded input and output.
$200
I would recommend the Fluval Sea over the Laguna/ATB or the Sicce pump. I have owned a Laguna and I'm currently using the Fluval sp6. It's a beast of a pump and makes zero noise. IMO it's the best submersible pump out right now.
If the Fluval is out of your price range, I'd go with the Laguna. But honestly Any of the pumps I listed are bad as$ pumps.