Who to Walk?

Chansey.

I'm in the same position. It's personal preference so I can only tell you what I'm choosing: I'm walking the Larvitar because my ~2900CP Blissey (P/DG) does okay in gyms, so to me getting a Tyranitar adds more to my enjoyment than a slightly stronger Blissey.

But I continue to hunt for 10 km eggs - just hatched a Chansey today in fact, so my Blissey is now at ~2950. And I'm hunting for places where Larvitars spawn - only seen 2 since start of gen 2... SAD!

I'm a lvl 32 trainer and my blissey is at lvl 31,5 right now. After maxing her out I will switch to Larvitar for sure. To get a second chansey and to max a hatched one out you would have to walk around 700 km because chansey is so rare.

If u want longevity in gyms, keep walking Chansey.

If you want increase your Pokedex entries and add another attacker to your current variety, then walk the Larvitar

In my case, I'd go walk with Larvitar. 2900 Blissey here doesn't last long. It's just a matter of taking over gyms when you can collect coins and don't expect them to stay for quite a while. A dedicated trainer with adequate resources will take down any gym whatever or whoever is defending it. Be it level 10 or what. Having a Tyranitar will help me in a lot of ways. e.g. having another quality attacker. That's just me.

It sounds like you are more likely to get more Larvitar candy from eggs, before you hatch a Chansey for candy.

So, I pick Chansey as your buddy