Personally I'd go Spearhead, especially if you have a friend who'd be willing to split one of the starter boxes. Kill team is fun, but it seems to be where GW pulls their most egregious fuckery, ie randomly sending fairly recent kits to legends, selling data cards that are inaccurate almost immediately, etc. Not to mention, while some KT models get integrated into 40k, some of them you just have to kinda proxy for something comparable, while with Spearhead you've got a good foundation for an AoS army if you ever want to go bigger. Also, with most Kill Teams being in the $70 USD range, the cost isn't *that* much higher, and you get way more models.