Unless the OP would also like advice on how to keep their costs to a minimum (admittedly they didn't mention this but who doesn't
)
Without any local knowledge, I would hazard a guess that those trains are rammed by the time they get to Manchester but not so bad from Helsby. In particular the 0721 starts from Chester so I'd expect it to be the quieter of those two.
edit:
I can't see any better fare than a Helsby-Manchester season from a quick look, though there may well be ways to get it cheaper - in particular if you require any other transport at either end there may be tickets for this which could be combined with your rail ticket.
The usual season ticket advice is as usual: buy for the period you actually require so you're not paying for travel at weekend or on holidays when it's not used. Tickets can be issued for any period between one calendar month and 10 months (more than 10 months is equivalent to an annual ticket), so work out how long you need or just ask for a season valid until a particular date. Also remember that your season is valid on any permitted route and at any intermediate stations, and if travelling overdistance your need only buy a ticket for the portion not covered by your season - the train need not stop at the "changeover" station in this case.