Havent played sentinel2 but did play sentinel 1 and defender chronicles. Defender chronicles is better value for money IMO.
I would say both. personally, I like Defender Chronicles a little better coz it's not a pure TD game that make it stand out.
I recently bought both and I can honestly say I've played Sentinel 2 for about 2 hours and Defender Chronicles for about 20. The replay factor is magnitudes higher with the inclusion of the RPG element. Plus it seems the devs for Defender Chronicles are much more active in terms of releasing updates with new features (maps, items, game play additions, etc).