Vitamin D Levels

Both forms are converted to active vitamin D by the body.

However, studies have determined that cholecalciferol is about three times more potent than ergocalciferol in the conversion process.
The test numbers you mention refer to blood levels of the major circulating form of vitamin D (in units called nmol/L).
Your current level of 27 is indeed on the low end.
It was once thought that blood levels of at least 40 were needed for optimal bone health. Vitamin D (along with calcium) is required to prevent the bone-thinning disease osteoporosis.
More recent research, however, indicates that higher levels of 70 to 80 might be optimal. So your doctor is on firm ground in wanting to push these numbers higher.
I'm taking the "MS" you mention to be multiple sclerosis. Some research suggests that long-term vitamin D supplementation might help prevent multiple sclerosis in women. Though you already have MS, your doctor could be hoping that raising your vitamin D levels might help this disease in addition to its other benefits.
To raise your levels to current recommendations, you may need at least 1,000 IU of supplemental vitamin D in the form of cholecalciferol (D3). I'd suggest talking with your doctor about this.
The safe upper intake level for vitamin D is currently set at 2,000 IU.
Vitamin D is available in both multivitamin and vitamin D-only supplements. Standard multivitamin supplements supply 400 IU of vitamin D, now considered a low-end amount.
The catch is, the vitamin D in supplement products may be either ergocalciferol are cholecalciferol.
The Supplement Facts label typically just says "Vitamin D." So how to tell which form you're getting?
You'll have to peruse the "Ingredients" section of the label, which is not alphabetized. The tiny print on some products calls for a magnifying lens.
Look for either "vitamin D3" or "cholecalciferol" as opposed to "vitamin D2" or "ergocalciferol."
Source: Richard Harness