Hey Joe,
I think the term undulating is the official term... but I understand that it is on the mild side of this classification. A guy called SJPC has created an unofficial map, which shows the undulation profile... At first glance, it looks bad... but look at the vertical scale, and you'll see that the climbs look ok. He put a link into this thread... see the bottom of page 25 of this thread... the post from 10th October at 12,55pm
Hard to advise which marathon to go for. Have a look at Abingdon's undulation profile... it might not be quite as flat as you think... but if it's flatter, and if you're fixated on your time, then that has to be an advantage.
But Preston would give you an extra week to recover and it is the first ever Guild Marathon, and a once in 20yr event!
Then you've got other factors to consider... like practicalities of travel. Family or personal ties to the towns. For me, it's the town of my birth & schooling. So the emotional pull makes this unmissable. Good luck with your decision. (I recommend Lancashire... but I'm biased!)
Southport strider... sounds a near identical time to me (20.5 miles in 3:38 last week) but a calf muscle at this stage is bad. At least you've done the key training now, and if it recovers well, you won't have missed too much.... but really hoping things work out for you. Bad luck.