Most midline and up arrangers have good sequencers built in, too.
I guess a LOT boils down to... Just how GOOD of a keyboard player are you? There is a LOT of skill involved doing an entire production on a keyboard. Drum programming is a skill unto itself (to make it realistic takes some serious chops), then there is guitar programming (a nightmare to do well!), good basslines, idiomatic horns and strings, etc., etc..
An arranger can take a lot of the hard work out of making an impressive sounding backing track, with little more skill needed than just playing the right chord at the right time.
Be careful of listening JUST to workstation players... few of them have even TOUCHED an arranger in the past few years, and most of them are vastly better, sounds AND style-wise, than they were just five years ago...
If it helps, I will let you know a little about me... I work as a fulltime musician in Florida, I play live gigs either with my duo (with an acoustic guitarist, doing 70's folk-rock) or with full bands. I use a G70 Roland, and use it for ALL my live gigs, OMB or not... I have a Kurzweil K2500 at home, but that gets used less and less these days, and I never gig it (the G70 is SO much easier to get around on, and sonically, the two compare very well).
I also do extensive studio work in my area, I have a home post-production and MIDI composing setup, and work at several local pro studios, doing playing, production and arranging. I've been a full-time pro for nearly 35 years.
I think I DO know what I'm doing (at least, most of the time!)... And I'll tell you this - If you are a noob at all this, a decent arranger will get you up and running faster, and easier than anything else out there. As I said earlier, unless you are doing primarily electronica, an arranger is by FAR the better choice for you. You lose a LITTLE in the nuts and bolts of voice editing, but you gain immensely in the ease of use area.
I can do a 'rough' of a song on an arranger in ten minutes. On any workstation, that is several hours to get right! If you are a songwriter, you don't want to bog down your creativity obsessing over stringing drum patterns together, you just want to lay it down and move on. That is the arranger's forté...
May I make a couple of suggestions for models to look at and try...?
As a pianist, perhaps you might be comfortable on a 76 note keyboard, rather than a little 61...? In that case, give Roland's E60 a good looking at (you said you were looking at an E50 - I would recommend paying a little bit more for the E60).
If you can handle a REALLY light keybed, the Yamaha PSR S900 is an excellent value for money, with some incredibly good guitar sounds. Guitars are, IMO, the hardest thing to get right on a keyboard, and Yamaha's Mega Voice technology makes the automatic ones in the styles sound very realistic. But it is a 61
Korg have a brand new (not shipping yet) PA500 in the same price bracket (a 61) that may be worth a look, too. Especially if you are looking to do extensive voice editing. They are the best at this, but most of the time songwriting, the built-in sounds work fine without much tweaking...
But... in a nutshell - Doing electronica? Get a workstation. Doing more conventional style music? Get an arranger...
I've got both, but I play my arranger more and more, these days.
Hope this helps