The preservation of fruit and vegetables via freezing or heat treatment and canning inevitably leads to the losses of some nutrients. Some of them are stable, e.g. fibre and some of them are lost in the processing/storage e.g. vitamin C. When we talk about the potential benefits of the foods, we have to understand, that there no ultimate 'good' for everyone. The balance in the diet is the most important and some of the nutrients listed here may not be the main concern in your diet anyway.
Vitamin C content of peas starts to decrease soon after harvesting. Luckily for us, both canning and freezing of this food take place pretty much as soon as peas are picked up. The difference between losses of the vitamin during processing is significant: tinned peas lose 73% of Vitamin C vs. 51% of losses in frozen (ref).
B vitamins losses are higher in canned peas than peas, cooked from frozen. Long story short - peas are not the main source of these vitamins anyway, apart from thiamin (B1). You can easily get it from other vegetables. But if you need to increase the intake of B vitamins in throughout your diet - get canned peas. More information on B vitamins is available here.
Salt (Sodium) content of canned peas is higher (almost two times) than frozen. That's because peas are usually preserved in salted water. Something to consider if you wish to reduce your salt intake.
Fibre content does not differ much between frozen and canned peas. However, the canning process, which involves heat treatment, makes fibre more soluble. This results in fibre being more useful for the organism and for microorganisms in the colon. Read more about the benefits of fibre here.
Antioxidants activity is much higher in frozen peas than in canned alternative. Antioxidants are compounds, that may prevent cell membranes damage. However, the benefits of antioxidants are under the debate between researchers and more studies needed to establish the potential benefits for the body.
My personal opinion: any peas a better than no peas at all. The research in this area is extremely limited by the variety of food processing (which changes rapidly) and the ways of storage and cooking. For this answer, I used the main reviews in the area and did not include conflicting results.
Three heaped tablespoons of both frozen and tinned peas will count as one of your 5-a-day and still be a healthy (and cheap!) option to have as a part of your diet.