To me, an AL 19 has never made any sense, when an AL 30 does everything better for a marginal increase in size and weight.
The difference between an AL 19 and an AL 30 is a half inch in diameter and 3" in length, and none of that is noticeable in the water, regardless of how you carry it. The AL 30 is about 3 pounds heavier overall out of the water (11 pounds versus 8 pounds) but it is .4 pounds less negative in the water (-1.0) when full.
The AL 30 also has the advantage of holding 30 cu ft at 3000 psi, so a button gauge allows you to easily assess remaining gas - 2,000 p= 20 cu ft, 1500 psi = 15 cu ft, etc - even if you are seriously math challenged.
And with 30 cu ft it has enough capacity to provide a more than adequate reserve for pony use, even if it isn't completely full.
Strangely enough I've been involved in technical diving for over 20 years now, and we primarily cave dive, usually encountering deco on almost every dive, unless it's very shallow cave - and I've never seen the need to own a AL 40.
Thus I'm surprised to hear "you really need a 40" if you move into tech. Well...surprised isn't the word as you hear that a lot, but it's perhaps a little disconcerting that so many technical divers fail to think it through and just accept it at face value.
Looking at it objectively, an AL 30 is large enough to be a useful deco bottle with enough O2 capacity to provide for a fair amount of deco at 20' plus a1.5x reserve.
Even with a fairly high SAC rate of .6 cfm on deco, it will allow for a full 20 minutes of deco at 20' with a 50% reserve, or if you complete the last stop at 10' about 6 minutes at 20' and 18 minutes at 10 ft.
With a better deco SAC of .4 cfm, the numbers rise to 31 minutes at 20 ft with a 50% reserve, or 9 minutes at 20ft and 25 minutes at 10 ft with a 50% reserve.
If you're getting into more deco than that, you should be doing a two deco gas dive, in which case you'll quite often use the first deco gas as a travel gas if it's a trimix dive, so you'll be using an AL 72 or an AL 80 for that gas and you still won't need a 40.
And if you're doing an offshore trip with limited access to O2 fills you'll want an AL 72 or AL 80 for your O2 bottle anyway, so once again an AL 40 isn't really needed.
Now, an AL 40 makes perfect sense for a technical diver who doesn't have an AL 30 when he or she starts technical diving and the extra 10 cu ft certainly doesn't hurt. However with the AL 40 being once again a half inch larger in diameter, 3" longer and 4 pounds heavier than an AL 30 - a total of 5.25" in diameter, 24.75" in length and 15 pounds in weight, AN AL 40 is arguably too large and heavy for a pony bottle, so it's hard to advocate for an AL 40 for a recreational diver. And the AL 40 feels larger in the water than an AL 30.
So don't discount the AL 30 for technical diving as it fits quite well into both single and two gas deco dive plans.