The key things to look at are the protein content and the minerals and vitamins. With in-lamb ewes you also want to be sure they have enough glucose in the final 6-8 weeks, especially if they are carrying multiple lambs.
You probably feed an 18% ewe roll, the Allstock is likely to be 16%. (These are my guesses - check with the feed merchants before making your decision.) Depending on your circumstances, 16% may be fine, or maybe you'd want to feed more of it (possibly negating any saving on purchase price per kg) - tell us more about your flock, when you lamb, what tup, how many lambs you get, what you usually feed, etc.
Mins and vits are very important to in-lamb and lactating ewes. So check what mins and vits and what %ages are in each of the feeds - you can always give mins and vits a different way (licks, blocks, powder, drench, jag) if needed; there are pros and cons of every method. But again, if you'd be giving extra mins & vits on top of the Allstock feed it could negate savings - although jag or drench does ensure that all ewes get a full dose, whereas any self-service method could mean some ewes get less than others.
feldar is right you need to check the suitability of the feed for tups. Some feeds can cause calculi in the urethra in tups, so if you'd be giving the feed to your tups do check with the feed merchant that it is suitable for tups. Our local merchant has adjusted their everyday 'stockblend' feed to be ok for tups, which has saved us all quite a bit of money and management!