This or the HR1290WFR, given they are the same size?

Decide on how long the string stays in. The HR1290WFR delivers 90.0 watts per cell at fifteen minutes against 80.83 here, so it is the stronger block by about ten percent and it costs less to buy. This one is rated to eight years of standby service against the HR's five, and carries a two year warranty against the HR's one. On a cabinet that gets opened every five years anyway the HR is the sensible choice. On a string you intend to leave alone for a decade, paying once for the Long Life plate costs less than paying twice for labor.

Does the lower internal resistance matter in practice?

It matters for testing more than for output. At 9.00 milliohms against the HR1290WFR's 9.9, this block starts life with a lower ohmic baseline, and conductance testing works by watching that number drift upward over the years. A lower starting point gives you more room before a reading crosses the threshold your maintenance program treats as end of life. It does not translate into more runtime; the watts column does that.

Why does the smaller block carry the same 300 amp limit as bigger ones?

Maximum discharge current follows the terminal and the plate group. An I1 insert on an M5 bolt is rated for what it is rated for, whatever sits behind it, which is why this 20 Ah block and the 23 Ah HR1290WFR share 300 amps while the 37 Ah HRL12150WG on a larger M6 insert is rated 225. Use the figure to check inrush, not to compare battery sizes.

Is the flame retardant case included?

Not on this part. CSB's datasheet describes the container as ABS to UL 94-HB with the 94-V0 flame retardant version available on request, and the price list lists this one as HRL 1280W with FR as a surcharged option rather than standard. Most of the HRL band ships FR as standard, so it is worth naming explicitly on a submittal. Ask a specialist to confirm the build in writing where the container rating has to be cited.