should it be invested in bitcoin and hold?
or
should it be invested in alt coins?
and why or what will make it sustainable via etc vs. alt investment
It really depends on how much risk you can handle and how active you want to be in trading..
Have you got enough hours in the week for trading, researching coins, keeping up with news etc.?
Or do you just want to invest and forget?
There is great potential to make huge, quick profits with altcoins but usually this requires you be actively trading, watching the markets daily, keeping up with news etc. As quickly as Altcoins can grow your profits, it can shrink them just as quickly too. Whereas Bitcoin is a steady grower. You can pretty much set and forget and you can be sure that in 12 months, your profits will be there.
Personally, I keep about 50% of my portfolio in Bitcoin & Ethereum, 30% in strong Alts like NEO, Lisk, Nem, Iota, Stratis, Ark, Qtum and about 20% on ICO's I've taken a gamble on like TenX, Vibe, REAL, PowerLedger etc..
I wouldn't recommend putting ALL your money into Altcoins.. That's a pretty huge gamble.
Whatever you invest in, make sure you take the time to learn about what you're doing so you can have the best chance of success.
Otherwise you're going to shit yourself and panic sell whenever the markets take a dip..