I don't have any storage on my bike right now except a small saddlebag under the seat. I'd like to add a rack and panniers/trunk bag, to fit a shirt, pants and shoes. What's best, a trunk bag or panniers?

Trunk bag with panniers!

https://shop.sunrisecyclery.com/item/12553/

Roll the pants and shirt.

Panniers that will allow you to carry heavier or larger loads.

For me, it depends.

I used to work at three locations around the city, and didn't have a locker at any one of them. So I had to carry that day's clothes and lunch with me. A trunk bag was just fine for that.

Now I work my entire week at one location and I have a locker. I use panniers on Monday to haul in the entire week's worth of work clothes, lunches and snacks. On Thursdays (four-day work week) I use the panniers to haul home the laundry. In between I can ride any bike I want without having to be concerned with hauling anything.

A trunk bag holds much less than a single pannier.

Just sayin'.

Do you need to haul lots of stuff? = Panniers
Not as much and lighter bike? = Trunk Bag.

I use a trunk bag. Panniers tend to make the bike pretty darn heavy.

Kinda depends on the size. I've seen some trunks with the drop down panniers that were equal or greater than some of the smaller panniers.

@OP- Other than the clothes and shoes, what else do plan on carrying? Rain gear? What about lunch? There's also another option you could explore, since you don't have a rack yet- Carradice.

I have a trunk bag with zip-out mini-panniers for most days. I also have a couple of large open-top grocery panniers when I need to carry more.

Panniers give you more flexibility. You can use one or both depending on what you're carrying. Plus you leave the top of the rack free in case something comes up and you need to strap something on there. I always have at least a bungee attached to the rack for strapping down any extra luggage.

You can bungee a shoebox to the top of your rack, they are about the size and shape of a trunk bag but are significantly lighter. I suggest new balance shoe boxes because the lid is attached to the box and not a separate piece that you can lose. They can be a little flimsy though but given the weight that you can carry in a trunk bag, they will probably suffice. If you want one that will hold up in the rain, you can collect enough corrugated plastic signs on the the side of the road and build a shoebox sized box out of that stuff. Either option will provide your bike with an extra ghetto look to it.

If you don't want to add a rack another alternative would be to add one of those carradice bags with a quick release bagman add on. they have mid sized ones that are easily 15L of space.

This is what I do too, I use panniers to carry a few changes of clothes and lunch fixings, and then I can use a small trunk bag for small things the rest of the week.

Thanks for the replies. I just realized I should have included a laptop in the list of potential cargo. Now that you mention it, I'm not sure if that automatically means I need panniers.

On the bike with the rack, the trunk bag is a permanent fixture. The panniers get added when I need the additional capacity.

I've not personally seen a "trunk" bag that will hold a laptop, but then I haven't looked for one. I'm sure one of the larger saddle bags would hold one, but it doesn't seem ideal. I would personally want the weight of a laptop either in a pannier or strapped to my back.

Both and then some. For big load days I go panniers. For small or medium loads I go trunk with drop down panniers. Depending on the destination or any planned stops where I might want to just carry one bag in the store or something, I've got a large handle bar bag that I use as a pannier more often than I mount it to the handle bars. It's easier to carry around somewhere then a set of panniers or my trunk bag. If I know I'm going to be off the bike with a medium to large load, I've got a Nashbar backpack than can convert to a pannier.

Books, notebook computer, or shirt on a card = pannier.

Rolled clothes, cable lock, or prescription pick-up = trunk bag.

Panniers on the 'mule' and trunkbag on my fg. I've got lockers at work that afford me the luxury of hauling a weeks worth of clothes/food in on Monday on my multi-speed commuter/tourer and bringing the days worn clothing home everyday on my fg. It's equipped w/a seatpost rack and a trunkbag. I keep a pair of work shoes in my locker that I spray w/bacteria killer everyday. Keeps the odor down real well.

I often use a trunk bag on a seatpost rack due to my multimode commute, panniers would make the bike too wide to easily fit on the bus rack with other bikes.

I have the topeak mtx trunk bag with panniers and while I don't need to carry a change of clothes, if i did I'd put them in the panniers and my food in the trunk. It's a nice system that slides on and off the bike easily, no fumbling with bungees.