Congrats on the split stay at TDR. We didn't transfer, but I can talk to the 'which park' part. If it's your third day in the parks, you'll be able to park hop; however, I'm not sure I'd strongly recommend it. The first two days on a three or four day ticket must be park specific (TDL / TDS or TDS /TDL). It's my experience that you'll have so much to see and do at either park that hopping is a waste of time. The folks at Oriental Land Co. have worked this and out make it mandatory or park guests, except for TDR owned and Good Neighbor hotels have a 3 or 4 day hopper ticket for an additional charge. Resist the urge as again you won't have the time or want to hop the first two days for a first time visit. I'd use the 3 or 4 days to go back and finish your shopping, eat favorite snack and ride Pooh's Honey Hunt again.
It really doesn't matter which park you see first. Some like Disneyland as its familiar while others can't wait for the new experience of Disney Sea. You'll have direct access from the hotel lobby into Disney Sea from MiraCosta. You'll enjoy that aspect of your stay. Maybe you'll go to Disney Sea if its not day 3/4 for the convenience. If its a hopper day, then is really doesn't matter. Enjoy the adventure!