If you can reduce your insulin levels that should promote fat and weight loss. Weight loss when you use insulin is harder. I understand your desire to lose weight and metformin is reasonably well accepted as a way of increasing insulin sensitivity and achieving weight loss for T1s. Actually other medications such as Actos also have been used with T1 as well.
In my view, if you don't have stomach issues then you should choose between Met and Met ER based on other considerations. First, Met reaches peak levels 1-2 hours after you take it while Met ER takes about 7 hours. This means that Met is best for timing to address meals and Met ER is best to address fasting levels. So a key questions is which do you think is causing you to use more insulin to drive your blood sugars into normal range, after meal or fasting?
The second consideration is max dosing. The max dose recommended for Met is 2500 mg, but for Met ER max dose is 2000mg. You can actually take more Met which may give you better results.
If you don't have gastric issues it certainly would be worth trying Met. You should take it 1-2 hours before your meal to achieve the greatest benefit.