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Which light bulb is more important UVB and UVA as have just got a pair of terras last year.

I still have much to learn about them and I want to ensure they have a good home to stay

UVB bulbs will also provide UVA, however a UVA bulb doesn't produce UVB.

Have a proper UVB bulb, and a heat lamp if needed, both above the basking area.

I agree with PokePony !
When it comes to UVB deal with a reputable brand. Some say for turtles but only offer very little UVB rays like 3% or less which is inadequate for a slider. One 5% of higher is best for a slider.
Also only a few brands provide both heat and UVB be careful. Like CFL type UVB's do not produce heat so an UVA heat bulb would be needed. Myself I found Mega Rays ( does both , it's a "MV" type) are the best , cost more up front but produce UVB much longer costing less in the long run !