I reckon if you can make Swan-dris on the machine, it's probably pretty powerful!  I suggest you take the big machine to Mr Singer - I reckon it's always worth investing some money in getting them serviced before a big job.  It probably hasn't been touched for years and with several things all not quite right, can simply be a nightmare to use.  They are quite precise pieces of machinery and the professionals can work magic on them. 

In the meantime, use the small one and avoid anything when it seems to struggle, in the hope that the big one will come back good as new.  If it doesn't seem to struggle, if you keep the bobbin holder surgically clean and change your needles regularly, the small one should cope.  If it really doesn't want to go through some very thick build up of cloth, you can wind the wheel by hand to punch the needle through.  Tedious, but not as tedious as doing it with needle and palm!  As long as you are considerate to the smaller machine, it will do its best for you.