Okay, so I want to create a display of multiple linear F20T12 bulbs (of vintage non-Eco GE just for consistency) to illustrate the different colours.
I was wanting to run them with Advance ballasts (just because I am familiar with the brand, not sure how good they are in relation to other brands for fluorescents).
I've come up with several types that seem to be compatible with this size, and I'm wondering what are the advantages, disadvantages of using them.

LC-14-20-C
Preheat ballast, runs 1x 14, 15, 18 or 20W linear bulb.  120V, draws 0.36A
obviously I'll need a starter to use this as well.

L-220F
Preheat ballast, runs 2x F20T12, 50°F min start temp.  120V, 0.55A line current.
requires 2 starters.

HM-1P20-TP
Preheat lamps in Rapid start circuit, runs 1x F20T12, 0°F min start temp.  120V, 0.24A line current.

HM-2SP20-TP
Preheat lamps in Rapid start circuit, runs 2x F20T12, 10°F min start temp.  120V, 0.48A line current.

RLQ-120-TP
Rapid start ballast, runs 1x F20T12, 50°F min start temp. 120V, 0.55A line current.

RL-2SP20-TP
Rapid start ballast, runs 2x F20T12, 50°F min start temp.  120V, 0.44A line current.

Some questions:  I'm wondering why the line current varies so much between ballast types?
Also, a rapid start ballast running a single tube draws more current than running 2 tubes? 

Which ballast runs the tube closest to F20T12 specs?  I assume this will mean it will maximize the life of the tube since fluorescents don't like to be under- or over-driven.

I don't have any particular preference, if preheat is best it would be neat to use starters, but if rapid start is best for F20T12s, then it would be more convenient.

The large variation in the current is because some of the ballasts you mentioned are high power factor and others are normal power factor. If you have two ballasts, one HPF and one NPF, that both run the lamp(s) at the same brightness, the HPF one will have a lower current rating.

I would recommend the LC-14-20-C or L-220F (which is basically two LC-14-20-C units in one case) if you want preheat, or HM-1P20-TP or HM-2SP20-TP if you want rapid/trigger start. The RL/RLQ ballasts significantly underdrive the lamps.

What matters is the lamp current, with the F20T12. Not the line current or with different lamp.
The "10/14/15/20" chokes can not run all the lamps (varying from 0.21 to 0.37A in current ratings) within specs, so the "0.36A" (in series circuit obviously is the lamp and line currents the same, but that is just for the simple series reactor) may be valid just the lamp with the lowest arc voltage (probably 14W or so), the 70V F20 will be driven way lower, so quite far below the spec.

With the RS ballasts you really have to take a clamp meter (what matters is the arc current and that is somehow split between the two wires feeding that filament; because of phase shifts, the straight sum does not work either) to get the lamp current.

With NPF autotransformer types you may estimate the arc current by:
- Measure the line current when open load (no lamp)
- Measure the OCV of the ballast
- Measure the line current with the desired lamp
Then the LampCurrent = (LineWithLamp - LineWithoutLamp) * 120V / OCV
But this is only approximate method, as it neglests few things (the line currents with and without lamp are in reality phase shifted a bit,...), that may influence the result. But without actual lamp arc current measurement you can not get anything better.

The Advance LC-25 (rated for F25T12 lamps) runs the F20T12 at close to spec.

Thanks very much for all the replies, I will stay away from the rapid start ballasts then.  I'll most likely go with double-tube ballasts to save weight and costs, since this display will have more than a handful of tubes in it as I add more colours to it.
Not sure if I'll go with preheat or preheat in rapid start yet, but still in planning stage yet.
Cheers!