3-4: This is a really good scheme if run in an aggressive fashion (i.e., not Fangio's design). In order to effectively run this scheme you must have (1) three very large and strong d-lineman b/c in the 3-4 their job is merely to occupy lineman; (2) a very good pass rusher at the OLB position; and (3) an effective blitzing DB.
If you look at the teams that run this scheme really well this is true:
Patriots- Wolfolk and Seymour make the line, Colvin is their rushing force w/ a very strong MLB corp, and before his injury Harrison was a great blitzer.
San Diego- there d-line is underrated, Merriman is a beast, and they have multiple dbs that excel at the blitz.
Pittsburgh- Casey Hampton (ex-Longhorn) is the best nose tackle in the league, Joey Porter is a good blitzer, and Polu...(however you spell it) is an amazing blitzer.
4-3: The greatest advantage to switching to this scheme is the players' general knowledge of the defense. Every player has played their defense throughout their career before coming to the Texans. Draft picks and young players can adapt more quickly to this defense and make a unit at least mediocre quickly.
I personally would continue using the 3-4 in a hybrid form for at least another year, as we will not be able to fully correct the offense (line, TE, Bush, etc.) in the same draft as selecting new players for the 4-3 scheme. We have to realized that other than our d-line, the defense is really young. Babin, Peek, Greenwood, Johnson, Robinson, Earl, Brown, Buchanon all have less 3 or less years in the scheme (and in many cases in the league as a whole).
As far as personnel drafted to fit the 3-4, I would generally agree the Texans have attempted to do this. However, Johnson is not properly suited for the scheme - he is a prototype DT in a 4-3 scheme. Additionally, Babin is more suited for the 3-4.