It's not really the size its the wattage, so what wattage do you want?
Both will essentially heat the same area given that unless your heating a non heat conductive material the material will heat up and spread heat in the general area.
A lot of people use two strips of 3" for larger cages and those kept in cooler rooms. So what condition does yours meet, large tub or cool room? Otherwise they are both essentially the same ones just more expensive to purchase and to operate.
Good Luck
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Jeremy
"I am become death, the destroyer of worlds" July 16, 1945 Robert Oppenheimer
1.1 Double Het "Sharp" Snow RTB's
1.1 Hypomelenistic RTB's
0.2 Pastel Hypo RTB's
2.0 Double Het Stripe Albino RTB's
0.1 Suriname RTB
0.1 Anerthrystic RTB
0.0.10 Red Bearded Dragons
1.1 Rhinoceros Iguana's
1.0 Green Iguana
1.0 Ball Python
1.1 Cream Golden Retrieviers
1.0 Pomeriaian
0.2 Catus Terribilis
0.1 Spouse
1.0 Child