Personally I would do both if you can because they are different tests. LT is for muscular-exertion and VO2-max is for aerobic-capacity. Having both sets of data and being able to graph relative to HR will give tremendous insight. However, as previously mentioned, you can test LT out in the field while VO2-max requires a lab. So if you can only do one, do the VO2-max first. While it does improve slightly with training, it doesn't increase anywhere nearly as dramatic as power-at-LT and muscle-efficiency (power per O2 consumed).