A lot of the time that will come down to personal preference, but in UU (I assume that is where this is for), beating out base Speed 80 Pokemon is a big advantage. That said, it can't actually get past many offensive base Speed 80s without a Jolly nature (Venusaur, the most common base Speed 80 in the tier will always be running a speed boosting nature unless it is a defensive set, in which case the speed will be negligible).
With this in mind, it may seem a little clearer, but if an offensive set is what you are after, then Speed is not something that you should be looking over unless it is absolutely necessary, so in the end it may be easier to pick a point that you can reach with the Speed you have (even if that is outrunning neutral base 80s) and then placing the rest in HP, most of the Pokemon that run Seismic Toss in UU will not stay in on a Kangaskhan anyway (Blissey and Registeel are the two biggest users of Seismic Toss and both are usually geared to taking special attacks and will not particularly like take an attack from Kangaskhan, espeically if you are running a Fighting move).
So, each has it's merits, but in the end it comes down to what you really want to do with it. If you want it to sweep, go with the speed, it will be more helpful in the long run, if you want to take a slower approach to things, invest in the HP, but know it is generally unneeded.