If you only had the 3DS, I'd say it was worth a buy. Currently, the 3DS's catalog is limited and contains mainly of rehashes of same old concepts - and you can get the games they mimic and more running on the Shield. It just isn't worthwhile to buy that console.
On the other hand, the Vita can and does emulate PSP and PS1 titles, plus it has its own miniscule library of games. When it will be hacked, it could potentially emulate NES, SNES, Gameboy\Color\Advance, and more. Having it as a POTENTIAL retro gaming system plus a new console may be a smart choice, and if you already own it - you may want to skip the Shield, as it will hold its value as a primary retro console for a limited time.
The pros of the Shield are already stated here - it runs Android and is compatible with most, if not all, of its library of games. It can function as a tablet. It is the most capable emulation and gaming Android device on the market. You really can't go wrong buying if you're looking for that kind of thing - but here's the catch. Being an Android device, it will not have exclusives, not major ones at the very least. So it isn't about broadening your horizons and experiencing new games.
To summarize, I think that getting a 3DS is a mistake. The Vita has untapped potential and is a good investment. The Shield is already proven as a gaming device, yet you only get what you see, and nothing more.
It's up to you to decide if you want another device, though personally, I think it's redundant.
