Depends what you run, and how many you run at once..
For (most) people, running DC, they will only run a couple of loco's at once. As long as they are not both Mogul's, 'Orient' sets, or big American outline diesels, then the smaller unit will cope, and the 15V upper limit will not be restrictive to top-speed.
You may, in the great scheme of things, need to run a separate supply for accessories, building lighting etc. Though I would suggest this is a good thing, as a fault in the 'accessories, will not stop you running trains.
But like storage

if you can stretch the budget to the larger unit, it will serve you well, and may be the only transformer you will need to buy?