The Sentinel will probably be a much better version of the Terrapin. I don't know why people are proposing the Turtle is a mining ship. It is a long range radar and scanning/exploration ship. If you look at the link Sam posted above at time index 22m, you'll see this. Freeze the vid at time index 22:40 and you'll see the detail that is has S2 radar, S2 scanners, S2 shields, etc. This is the first time I have seen size designations for scanners and radar, but you should expect this kind of utility in the Sentinel. The difference is the Sentinel is not a starter ship. Its a GOOD ship that probably outperforms the Turtle in every way. It should. It costs more. The Lancer Dur is a much more capable ship than the terrapin save that it will probably not be nearly so stealthy, and stealth is a real advantage when scanning and exploring.
BTW, did y'all notice it in on the official ship comparison page? It has a single S5 unmanned turret. Not shabby for a starter ship, but they're calling S2 shields "heavily shielded", which is a bad joke. That thing is a flying target, but probably quite stealthy when wearing void armor.