I decided to vote for the first option.
I know people are going to wonder why, and to be honest? It's a tough call. The 'inspiration' games behind Grip are unique in their shield mechanics compared to games of similar genre, which sets it apart, but with the way things currently are in terms of weapons and such having a permanent shield in the same vein might be making first place too easy to keep hold of.
It's a tough call because changing the mechanics of the shield can have a lot of knock-on effects for balance -- I understand this -- but we all here should know how the shield behaves as it is and at the very least I feel we should all try and see how the game fares with a shield that lasts for only ten seconds but is going to deflect pretty much everything that gets thrown at it. For all we know it could vastly improve the balance of the game and make being in first place more challenging (and not just a target for assassin spam). Or it could end up being sucky and needs to be reverted back, but yeah.
As for slots, I feel the shield may need a permanent slot until it's depleted/destroyed/etc. Easiest way to have a button to flip the shield back and forth when that's implemented.
Look at the road-holding on these babies. Clarkson would cream himself. Broscar, your car skin is called Tropical. ; )