Get a software sampler.
Cheaper, smaller, way more comfortable (Halion reads many formats), more flexible, easy to update, offers better editing-possibilitys and has more options to alter the sound.
Plus, you can drop some plugins behind the samplers out.
You get free software sequencers, which are actually mostly better than any build-in sequnencer of a sampler.
If you can't let go of a real sampler, I'd recommend the Groovebox. The MC 505 from Roland. It's nearly a small workstation.
You'll got a step-sequencer and a synthesizer and a drummachine in one piece. And you have an extreme funky toy on it, the D-Beam COntroller (silly name), which allows you to alter the sound by just waving your hand over the controller. It's all controlled by the wind you make over it. Looks great in live-shows, believe me... Or get drunk and hang out a night over that machine, I bet you've got amazing results the next morning.
You can also buy the MC 303, the baby-brother of the 505, it just lacks the D-Beam Controller (total pity) and some sampling-space if I remember correctly.
It's nearly the same with the SP-505 and 303 you talked about.
The Boss-machines are a bit more developed, the 505 for example has a bigger LCD-screen which allows you to actually see the envelopes you create. (Which you will never need, rely on your ear, man.)
But I find that the ROland machines sound a bit more grunty, if you want them to.
It's a question of taste, I can't help you with that. Borrow the machines for a weekend and chek them out yourself.