In personal terms, I’d say Shadow of the Wolf is definitely my favourite of the two, but I’m not going to say something so broad and universal as “it’s the best starting module for Lancer.” It’s the module I would choose if I were introducing people to Lancer (and, indeed, I’m currently running it for a bunch of new players), but it all depends on what you want.

Shadow of the Wolf is very character-focused and leans heavily into the deep lore of Lancer, which is good if you want a roleplay-heavy adventure with lots of memorable characters and an interesting setting. However, your players really do need to be heavily invested in the elevator pitch of Shadow of the Wolf to get the most out of it- you need to be interested in the idea of going to Neofeudal Mech University and you need to have a character who can interface with ideas of classism, elitism and entrenched hierarchical traditionalism.

Solstice Rain is certainly more roleplay-lite on its face - it’s military fiction played very straight. But it is certainly less daunting to make a character for; you don’t really need to understand very much of the setting’s lore. It’s made by Kai Tave, who’s a mastermind at Lancer design, so the battles are challenging, the exotic gear is well-tuned and everything just fits together very well. You can pick up Solstice Rain, roll basic LL0 characters and be fighting mechs inside of 20-30 minutes, and it does a great job of teaching players how the game works by making them experience it.

I honestly think people should buy both books, and offer both of them to their players. Kai and Kat both deserve your money.