this might make me seem intelligent, but i can assure you i'm not, this came from Karl Norris

200 blocks have several differences to a 205, but the three most important that give considerable increase in strength/improvement are:

1. Extra thickness to the bore tube up to half bore tube height. The bore tubes are also siamesed together unlike the 205 block.

2. Full height cast in pillars for no.3 pair of head bolts. This means the land into which no.3 pair of head bolts screw, is a full height cast internal pillar, adding considerable strength to the deck face, and is why long studding is NOT necessary.

3. At the base of the water jacket are 10 square cast bosses which allow the block to be 10 studded if required. (This process I carry out only when block cracking is an issue around no.2 or no.4 head bolt pairs. However from experience the block cracking issue around no.2 and no.4 pair of head bolts is actually a heat issue, and often associated with head gasket failure. It is NOT the daft misconception that the head lifting cracks the block due to using head bolts!

Other minor mods to the 200 block vary depending upon year of 200 block and include: (years are approx as I'm sure the exact date varies)

Deletion of rear core plug (1992 onwards) Designated V90HM 6015 AB (pre 92 blocks are V90HM6015AA)
Deletion of mechanical fuel pump drive machining (1994 onwards)
4 Additional internal bosses inside crank case for machining of internal piston cooling jets as used on EECIV escos (1992 onwards)
Use of larger size main bearing caps over early 205 blocks though these are also found on late 205 locks.

That folks covers all the significant changes to a 200 block and is as you can see quite significant. It does however not mean that 205 blocks are no use, simply that the 200 block is in design at least, a stronger block.