Depends on what you are looking for...... the cheapest bulk systems use open tanks and can be a bit messy along with introducing pressure problems in the ink train, the newer more expensive bulk systems are mostly all moving towards ink bag setups. These have the advantage of not introducing potential pressure issues on the ink train (cartridges also use ink bags) and have the added benefit of pretty much zero mess and never exposing the ink to air. When ink gets used (in a cartridge or in a bulk bag type setup) the bag collapses, with an open tank setup you have a rigid tank with the top closed. If that top is sealed, as ink gets used the tank cannot collapse which causes negative pressure, if its vented to prevent that then you have the potential for excessive positive pressure and the ink is getting exposed to air. Any way you look at it, bag type systems are superior.
Then you have the ink itself. Do you want so called "eco" or "lite" solvent inks or full solvent inks? Eco and lite solvent inks do not really mean ecologically friendly, just less odor, and more often than not, less durability. Full solvent inks have strong odor. You will also want to consider a brand that is readily available to you as well as costs including shipping costs if applicable.
We have been using Triangle JVV ink in our JV33 since near day one with excellent results. First using their cartridges and for a year and half or so using Triangles Eco Bulk system.
http://www.signs101.com/forums/showthread.php?92386
Love it! Works great, have not once spilled a single drop of ink using it since the ink is always sealed in bags. Their JVV ink is very strong though, I'd say stronger than OEM SS21. You definitely need ventilation.... We also create all of our own custom profiles using an i1. With our own custom profiles the JVV ink gives us a wider color gamut than oem inks as well. Ink costs using the 2 liter boxes we use.... per ml - we are well under half of the cost of OEM ink and as another added bonus we are not constantly throwing big plastic cartridges away. Triangle 2l inks are in cardboard boxes which is much more trash friendly.