If you already have the 151A on hand, I'd suggest putting it in, then removing it to see if it collapsed any. If so, go with shorter one, or get another brand such as K&N, Wix, Bosch, all good filters that fit properly...I've used them. The filter should fit snugly up in the cap.
My last reply to this . I called Mobile 1 Official Site today to see if they could answer the question. They said that 151 and 151A are the same with the exception that the "A" version is the new one and that the "O" ring is rated at a higher temp. (20,000 deg. (I think). Anyway he said they should be the same size and fit the same vehicle. I will check my "A" version in the AM to see if it has collapsed. Like @SlingLow ready to buy another brand.