“Letter Spacing” ≠ readability
The “spacing of the letters”, i.e. the overall tracking of the typeface, has a fairly minor effect on the readability of said face. Using a proper typographically-sensitive application like a page layout package, you can adjust the tracking over a wide range and until the glyphs are actually colliding or becoming so separated that they don't read as a unified whole, the difference in readability is very slight. Besides, you get a greater variation in overall tracking when you use full-justified text vs. left-justified/rag-right alignment than you would switching between two similar faces such as Arial vs. Helvetica. Long story short*, the effect of character spacing on readability is small compared to the design of the typefaces themselves.
And that's where, as others have pointed out, Arial earns a monumental fail. Its glyphs are really bush-league compared to the craftsmanship of Helvetica. Its ubiquity is a rematch of VHS-vs.-Beta; the crappier one won despite a four-decade lead by the better typeface. Who wants to put MS on trial for crimes against typography?
Now, in terms of saving ink, approaches like EcoFont are a silly idea. With the resolution of modern inkjet and laser printers, simply having a printer driver that substitutes a 90% screen on type will reduce ink usage for any typeface without a loss of readability (except in really tiny type). My 11-year-old HP LaserJet 6MP has been able to do this from the get-go... and I can also get ten reams printed from a single cartridge most of the time. Try THAT on an inkjet: you'll have about nine blank reams at the end, or you'll have paid five times as much in consumables.
So, if your goal is economy, buy a laser printer. Use the eco-printing mode when the purpose is documentation rather than perfect appearance. And don't print unless you have to.
* I know, too late now.