There is actually a big difference in the 2 machines. The biggest part is the head configuration. I believe the SC500 only has 2 heads while the SC545 has 6. This translates to speed. The SC500 prints 3 colors with each head while the SC545 has 1 head for each color.
The SC500 is rated at 13 sq feet per hour at 720x720 8 pass mode
The SC545EX is rated at 86 sq feet per hour at 720x720 8 pass mode
Yes both machines can use the triangle inks, but there are very few profiles for the SC500.
Honestly I would choose the Versacamm SP540V over the SC500 as it can print much faster at 32 sq feet per hour and has an abundance of profiles avaible. The SC545EX is about 10K more, but it is significantly faster.
The sq foot numbers above are what Roland has on their site. These numbers can normally be doubled when using the triangle inks and the provided profiles. Im printing about 56-60 sq feet an hour with my Versacamm SP300 even though Roland states on their web page that my speed should be around 26.9 sq feet per hour.